diff --git a/dlls/kernel32/tests/fiber.c b/dlls/kernel32/tests/fiber.c
index 1e4ca666853..9ff9366c993 100644
--- a/dlls/kernel32/tests/fiber.c
+++ b/dlls/kernel32/tests/fiber.c
@@ -33,9 +33,13 @@ static BOOL (WINAPI *pFlsFree)(DWORD);
 static PVOID (WINAPI *pFlsGetValue)(DWORD);
 static BOOL (WINAPI *pFlsSetValue)(DWORD,PVOID);
 
-static LPVOID fibers[2];
+static void *fibers[3];
 static BYTE testparam = 185;
-static WORD cbCount;
+static DWORD fls_index_to_set = FLS_OUT_OF_INDEXES;
+static void* fls_value_to_set;
+
+static int fiberCount = 0;
+static int cbCount = 0;
 
 static VOID init_funcs(void)
 {
@@ -59,15 +63,35 @@ static VOID init_funcs(void)
 
 static VOID WINAPI FiberLocalStorageProc(PVOID lpFlsData)
 {
+    ok(lpFlsData == fls_value_to_set,
+       "FlsData expected not to be changed, value is %p, expected %p\n",
+       lpFlsData, fls_value_to_set);
     cbCount++;
-    ok(lpFlsData == (PVOID) 1587, "FlsData expected not to be changed\n");
 }
 
 static VOID WINAPI FiberMainProc(LPVOID lpFiberParameter)
 {
     BYTE *tparam = (BYTE *)lpFiberParameter;
-    cbCount++;
+    fiberCount++;
     ok(*tparam == 185, "Parameterdata expected not to be changed\n");
+    if (fls_index_to_set != FLS_OUT_OF_INDEXES)
+    {
+        void* ret;
+        BOOL bret;
+
+        ret = pFlsGetValue(fls_index_to_set);
+        ok(ret == NULL, "FlsGetValue returned %p, expected NULL\n", ret);
+
+        /* Set the FLS value */
+        bret = pFlsSetValue(fls_index_to_set, fls_value_to_set);
+        ok(bret, "FlsSetValue failed with error %u\n", GetLastError());
+
+        /* Verify that FlsGetValue retrieves the value set by FlsSetValue */
+        SetLastError( 0xdeadbeef );
+        ret = pFlsGetValue(fls_index_to_set);
+        ok(ret == fls_value_to_set, "FlsGetValue returned %p, expected %p\n", ret, fls_value_to_set);
+        ok(GetLastError() == ERROR_SUCCESS, "FlsGetValue error %u\n", GetLastError());
+    }
     pSwitchToFiber(fibers[0]);
 }
 
@@ -76,7 +100,7 @@ static void test_ConvertThreadToFiber(void)
     if (pConvertThreadToFiber)
     {
         fibers[0] = pConvertThreadToFiber(&testparam);
-        ok(fibers[0] != 0, "ConvertThreadToFiber failed with error %d\n", GetLastError());
+        ok(fibers[0] != NULL, "ConvertThreadToFiber failed with error %u\n", GetLastError());
     }
     else
     {
@@ -89,7 +113,7 @@ static void test_ConvertThreadToFiberEx(void)
     if (pConvertThreadToFiberEx)
     {
         fibers[0] = pConvertThreadToFiberEx(&testparam, 0);
-        ok(fibers[0] != 0, "ConvertThreadToFiberEx failed with error %d\n", GetLastError());
+        ok(fibers[0] != NULL, "ConvertThreadToFiberEx failed with error %u\n", GetLastError());
     }
     else
     {
@@ -102,7 +126,7 @@ static void test_ConvertFiberToThread(void)
     if (pConvertFiberToThread)
     {
         BOOL ret = pConvertFiberToThread();
-        ok(ret, "ConvertFiberToThread failed with error %d\n", GetLastError());
+        ok(ret, "ConvertFiberToThread failed with error %u\n", GetLastError());
     }
     else
     {
@@ -112,9 +136,9 @@ static void test_ConvertFiberToThread(void)
 
 static void test_FiberHandling(void)
 {
-    cbCount = 0;
+    fiberCount = 0;
     fibers[0] = pCreateFiber(0,FiberMainProc,&testparam);
-    ok(fibers[0] != 0, "CreateFiber failed with error %d\n", GetLastError());
+    ok(fibers[0] != NULL, "CreateFiber failed with error %u\n", GetLastError());
     pDeleteFiber(fibers[0]);
 
     test_ConvertThreadToFiber();
@@ -124,12 +148,11 @@ static void test_FiberHandling(void)
     else
         test_ConvertThreadToFiber();
 
-
     fibers[1] = pCreateFiber(0,FiberMainProc,&testparam);
-    ok(fibers[1] != 0, "CreateFiber failed with error %d\n", GetLastError());
+    ok(fibers[1] != NULL, "CreateFiber failed with error %u\n", GetLastError());
 
     pSwitchToFiber(fibers[1]);
-    ok(cbCount == 1, "Wrong callback count: %d\n", cbCount);
+    ok(fiberCount == 1, "Wrong fiber count: %d\n", fiberCount);
     pDeleteFiber(fibers[1]);
 
     if (!pCreateFiberEx)
@@ -138,12 +161,11 @@ static void test_FiberHandling(void)
         return;
     }
 
-    SetLastError(0xdeadbeef);
     fibers[1] = pCreateFiberEx(0,0,0,FiberMainProc,&testparam);
-    ok(fibers[1] != 0, "CreateFiberEx failed with error %d\n", GetLastError());
+    ok(fibers[1] != NULL, "CreateFiberEx failed with error %u\n", GetLastError());
 
     pSwitchToFiber(fibers[1]);
-    ok(cbCount == 2, "Wrong callback count: %d\n", cbCount);
+    ok(fiberCount == 2, "Wrong fiber count: %d\n", fiberCount);
     pDeleteFiber(fibers[1]);
 
     if (!pIsThreadAFiber)
@@ -157,32 +179,62 @@ static void test_FiberHandling(void)
     ok(!pIsThreadAFiber(), "IsThreadAFiber reported TRUE\n");
 }
 
-static void test_FiberLocalStorage(PFLS_CALLBACK_FUNCTION cbfunc)
+static void test_FiberLocalStorage(void)
 {
-    DWORD fls;
+    DWORD fls, fls_2;
     BOOL ret;
-    PVOID val = (PVOID) 1587;
+    void* val;
 
-    if (!pFlsAlloc)
+    if (!pFlsAlloc || !pFlsSetValue || !pFlsGetValue || !pFlsFree)
     {
         win_skip( "Fiber Local Storage not supported\n" );
         return;
     }
-    cbCount = 0;
 
-    fls = pFlsAlloc(cbfunc);
-    ok(fls != FLS_OUT_OF_INDEXES, "FlsAlloc failed with error %d\n", GetLastError());
+    /* Test an unallocated index
+     * FlsFree should fail
+     * FlsGetValue and FlsSetValue should succeed
+     */
+    SetLastError( 0xdeadbeef );
+    ret = pFlsFree( 127 );
+    ok( !ret, "freeing fls index 127 (unallocated) succeeded\n" );
+    ok( GetLastError() == ERROR_INVALID_PARAMETER,
+        "freeing fls index 127 (unallocated) wrong error %u\n", GetLastError() );
 
-    ret = pFlsSetValue(fls, val);
-    ok(ret, "FlsSetValue failed\n");
-    ok(val == pFlsGetValue(fls), "FlsGetValue failed\n");
+    val = pFlsGetValue( 127 );
+    ok( val == NULL,
+        "getting fls index 127 (unallocated) failed with error %u\n", GetLastError() );
 
-    ret = pFlsFree(fls);
-    ok(ret, "FlsFree failed\n");
-    if (cbfunc)
-        todo_wine ok(cbCount == 1, "Wrong callback count: %d\n", cbCount);
+    ret = pFlsSetValue( 127, (void*) 0x217 );
+    ok( ret, "setting fls index 127 (unallocated) failed with error %u\n", GetLastError() );
 
-    /* test index 0 */
+    SetLastError( 0xdeadbeef );
+    val = pFlsGetValue( 127 );
+    ok( val == (void*) 0x217, "fls index 127 (unallocated) wrong value %p\n", val );
+    ok( GetLastError() == ERROR_SUCCESS,
+        "getting fls index 127 (unallocated) failed with error %u\n", GetLastError() );
+
+    /* FlsFree, FlsGetValue, and FlsSetValue out of bounds should return
+     * ERROR_INVALID_PARAMETER
+     */
+    SetLastError( 0xdeadbeef );
+    ret = pFlsFree( 128 );
+    ok( !ret, "freeing fls index 128 (out of bounds) succeeded\n" );
+    ok( GetLastError() == ERROR_INVALID_PARAMETER,
+        "freeing fls index 128 (out of bounds) wrong error %u\n", GetLastError() );
+
+    SetLastError( 0xdeadbeef );
+    ret = pFlsSetValue( 128, (void*) 0x217 );
+    ok( !ret, "setting fls index 128 (out of bounds) succeeded\n" );
+    ok( GetLastError() == ERROR_INVALID_PARAMETER,
+        "setting fls index 128 (out of bounds) wrong error %u\n", GetLastError() );
+
+    SetLastError( 0xdeadbeef );
+    val = pFlsGetValue( 128 );
+    ok( GetLastError() == ERROR_INVALID_PARAMETER,
+        "getting fls index 128 (out of bounds) wrong error %u\n", GetLastError() );
+
+    /* Test index 0 */
     SetLastError( 0xdeadbeef );
     val = pFlsGetValue( 0 );
     ok( !val, "fls index 0 set to %p\n", val );
@@ -196,6 +248,7 @@ static void test_FiberLocalStorage(PFLS_CALLBACK_FUNCTION cbfunc)
     ok( !val, "fls index 0 wrong value %p\n", val );
     ok( GetLastError() == ERROR_INVALID_PARAMETER, "setting fls index wrong error %u\n", GetLastError() );
 
+    /* Test creating an FLS index */
     fls = pFlsAlloc( NULL );
     ok( fls != FLS_OUT_OF_INDEXES, "FlsAlloc failed\n" );
     ok( fls != 0, "fls index 0 allocated\n" );
@@ -203,13 +256,149 @@ static void test_FiberLocalStorage(PFLS_CALLBACK_FUNCTION cbfunc)
     ok( !val, "fls index %u wrong value %p\n", fls, val );
     ret = pFlsSetValue( fls, (void *)0xdeadbeef );
     ok( ret, "setting fls index %u failed\n", fls );
+    SetLastError( 0xdeadbeef );
     val = pFlsGetValue( fls );
     ok( val == (void *)0xdeadbeef, "fls index %u wrong value %p\n", fls, val );
+    ok( GetLastError() == ERROR_SUCCESS,
+        "getting fls index %u failed with error %u\n", fls, GetLastError() );
     pFlsFree( fls );
+
+    /* Undefined behavior: verify the value is NULL after it the slot is freed */
+    SetLastError( 0xdeadbeef );
+    val = pFlsGetValue( fls );
+    ok( val == NULL, "fls index %u wrong value %p\n", fls, val );
+    ok( GetLastError() == ERROR_SUCCESS,
+        "getting fls index %u failed with error %u\n", fls, GetLastError() );
+
+    /* Undefined behavior: verify the value is settable after the slot is freed */
     ret = pFlsSetValue( fls, (void *)0xdeadbabe );
     ok( ret, "setting fls index %u failed\n", fls );
     val = pFlsGetValue( fls );
     ok( val == (void *)0xdeadbabe, "fls index %u wrong value %p\n", fls, val );
+
+    /* Try to create the same FLS index again, and verify that is initialized to NULL */
+    fls_2 = pFlsAlloc( NULL );
+    ok( fls != FLS_OUT_OF_INDEXES, "FlsAlloc failed with error %u\n", GetLastError() );
+    /* If this fails it is not an API error, but the test will be inconclusive */
+    ok( fls_2 == fls, "different FLS index allocated, was %u, now %u\n", fls, fls_2 );
+
+    SetLastError( 0xdeadbeef );
+    val = pFlsGetValue( fls_2 );
+    ok( val == NULL, "fls index %u wrong value %p\n", fls, val );
+    ok( GetLastError() == ERROR_SUCCESS,
+        "getting fls index %u failed with error %u\n", fls_2, GetLastError() );
+    pFlsFree( fls_2 );
+}
+
+static void test_FiberLocalStorageCallback(PFLS_CALLBACK_FUNCTION cbfunc)
+{
+    DWORD fls;
+    BOOL ret;
+    void* val, *val2;
+
+    if (!pFlsAlloc || !pFlsSetValue || !pFlsGetValue || !pFlsFree)
+    {
+        win_skip( "Fiber Local Storage not supported\n" );
+        return;
+    }
+
+    /* Test that the callback is executed */
+    cbCount = 0;
+    fls = pFlsAlloc( cbfunc );
+    ok( fls != FLS_OUT_OF_INDEXES, "FlsAlloc failed with error %u\n", GetLastError() );
+
+    val = (void*) 0x1587;
+    fls_value_to_set = val;
+    ret = pFlsSetValue( fls, val );
+    ok(ret, "FlsSetValue failed with error %u\n", GetLastError() );
+
+    val2 = pFlsGetValue( fls );
+    ok(val == val2, "FlsGetValue returned %p, expected %p\n", val2, val);
+
+    ret = pFlsFree( fls );
+    ok(ret, "FlsFree failed with error %u\n", GetLastError() );
+    todo_wine ok( cbCount == 1, "Wrong callback count: %d\n", cbCount );
+
+    /* Test that callback is not executed if value is NULL */
+    cbCount = 0;
+    fls = pFlsAlloc( cbfunc );
+    ok( fls != FLS_OUT_OF_INDEXES, "FlsAlloc failed with error %u\n", GetLastError() );
+
+    ret = pFlsSetValue( fls, NULL );
+    ok( ret, "FlsSetValue failed with error %u\n", GetLastError() );
+
+    pFlsFree( fls );
+    ok( ret, "FlsFree failed with error %u\n", GetLastError() );
+    ok( cbCount == 0, "Wrong callback count: %d\n", cbCount );
+}
+
+static void test_FiberLocalStorageWithFibers(PFLS_CALLBACK_FUNCTION cbfunc)
+{
+    void* val1 = (void*) 0x314;
+    void* val2 = (void*) 0x152;
+    BOOL ret;
+
+    if (!pFlsAlloc || !pFlsFree || !pFlsSetValue || !pFlsGetValue)
+    {
+        win_skip( "Fiber Local Storage not supported\n" );
+        return;
+    }
+
+    fls_index_to_set = pFlsAlloc(cbfunc);
+    ok(fls_index_to_set != FLS_OUT_OF_INDEXES, "FlsAlloc failed with error %u\n", GetLastError());
+
+    test_ConvertThreadToFiber();
+
+    fiberCount = 0;
+    cbCount = 0;
+    fibers[1] = pCreateFiber(0,FiberMainProc,&testparam);
+    fibers[2] = pCreateFiber(0,FiberMainProc,&testparam);
+    ok(fibers[1] != NULL, "CreateFiber failed with error %u\n", GetLastError());
+    ok(fibers[2] != NULL, "CreateFiber failed with error %u\n", GetLastError());
+    ok(fiberCount == 0, "Wrong fiber count: %d\n", fiberCount);
+    ok(cbCount == 0, "Wrong callback count: %d\n", cbCount);
+
+    fiberCount = 0;
+    cbCount = 0;
+    fls_value_to_set = val1;
+    pSwitchToFiber(fibers[1]);
+    ok(fiberCount == 1, "Wrong fiber count: %d\n", fiberCount);
+    ok(cbCount == 0, "Wrong callback count: %d\n", cbCount);
+
+    fiberCount = 0;
+    cbCount = 0;
+    fls_value_to_set = val2;
+    pSwitchToFiber(fibers[2]);
+    ok(fiberCount == 1, "Wrong fiber count: %d\n", fiberCount);
+    ok(cbCount == 0, "Wrong callback count: %d\n", cbCount);
+
+    fls_value_to_set = val2;
+    ret = pFlsSetValue(fls_index_to_set, fls_value_to_set);
+    ok(ret, "FlsSetValue failed\n");
+    ok(val2 == pFlsGetValue(fls_index_to_set), "FlsGetValue failed\n");
+
+    fiberCount = 0;
+    cbCount = 0;
+    fls_value_to_set = val1;
+    pDeleteFiber(fibers[1]);
+    ok(fiberCount == 0, "Wrong fiber count: %d\n", fiberCount);
+    todo_wine ok(cbCount == 1, "Wrong callback count: %d\n", cbCount);
+
+    fiberCount = 0;
+    cbCount = 0;
+    fls_value_to_set = val2;
+    pFlsFree(fls_index_to_set);
+    ok(fiberCount == 0, "Wrong fiber count: %d\n", fiberCount);
+    todo_wine ok(cbCount == 2, "Wrong callback count: %d\n", cbCount);
+
+    fiberCount = 0;
+    cbCount = 0;
+    fls_value_to_set = val1;
+    pDeleteFiber(fibers[2]);
+    ok(fiberCount == 0, "Wrong fiber count: %d\n", fiberCount);
+    ok(cbCount == 0, "Wrong callback count: %d\n", cbCount);
+
+    test_ConvertFiberToThread();
 }
 
 START_TEST(fiber)
@@ -223,6 +412,7 @@ START_TEST(fiber)
     }
 
     test_FiberHandling();
-    test_FiberLocalStorage(NULL);
-    test_FiberLocalStorage(FiberLocalStorageProc);
+    test_FiberLocalStorage();
+    test_FiberLocalStorageCallback(FiberLocalStorageProc);
+    test_FiberLocalStorageWithFibers(FiberLocalStorageProc);
 }