diff --git a/dlls/setupapi/stringtable.c b/dlls/setupapi/stringtable.c
index 59ad6ce1b86..e6578e7e7ce 100644
--- a/dlls/setupapi/stringtable.c
+++ b/dlls/setupapi/stringtable.c
@@ -189,98 +189,6 @@ StringTableDestroy(HSTRING_TABLE hStringTable)
 }
 
 
-/**************************************************************************
- * StringTableAddString [SETUPAPI.@]
- *
- * Adds a new string to the string table.
- *
- * PARAMS
- *     hStringTable [I] Handle to the string table
- *     lpString     [I] String to be added to the string table
- *     dwFlags      [I] Flags
- *                        1: case sensitive compare
- *
- * RETURNS
- *     Success: String ID
- *     Failure: -1
- *
- * NOTES
- *     If the given string already exists in the string table it will not
- *     be added again. The ID of the existing string will be returned in
- *     this case.
- */
-DWORD WINAPI
-StringTableAddString(HSTRING_TABLE hStringTable,
-                     LPWSTR lpString,
-                     DWORD dwFlags)
-{
-    PSTRING_TABLE pStringTable;
-    DWORD i;
-
-    TRACE("%p %s %x\n", hStringTable, debugstr_w(lpString), dwFlags);
-
-    pStringTable = (PSTRING_TABLE)hStringTable;
-    if (pStringTable == NULL)
-    {
-        ERR("Invalid hStringTable!\n");
-        return (DWORD)-1;
-    }
-
-    /* Search for existing string in the string table */
-    for (i = 0; i < pStringTable->dwMaxSlots; i++)
-    {
-        if (pStringTable->pSlots[i].pString != NULL)
-        {
-            if (dwFlags & 1)
-            {
-                if (!lstrcmpW(pStringTable->pSlots[i].pString, lpString))
-                {
-                    return i + 1;
-                }
-            }
-            else
-            {
-                if (!lstrcmpiW(pStringTable->pSlots[i].pString, lpString))
-                {
-                    return i + 1;
-                }
-            }
-        }
-    }
-
-    /* Check for filled slot table */
-    if (pStringTable->dwUsedSlots == pStringTable->dwMaxSlots)
-    {
-        FIXME("Resize the string table!\n");
-        return (DWORD)-1;
-    }
-
-    /* Search for an empty slot */
-    for (i = 0; i < pStringTable->dwMaxSlots; i++)
-    {
-        if (pStringTable->pSlots[i].pString == NULL)
-        {
-            pStringTable->pSlots[i].pString = MyMalloc((lstrlenW(lpString) + 1) * sizeof(WCHAR));
-            if (pStringTable->pSlots[i].pString == NULL)
-            {
-                TRACE("Couldn't allocate memory for a new string!\n");
-                return (DWORD)-1;
-            }
-
-            lstrcpyW(pStringTable->pSlots[i].pString, lpString);
-
-            pStringTable->dwUsedSlots++;
-
-            return i + 1;
-        }
-    }
-
-    TRACE("Couldn't find an empty slot!\n");
-
-    return (DWORD)-1;
-}
-
-
 /**************************************************************************
  * StringTableAddStringEx [SETUPAPI.@]
  *
@@ -296,7 +204,7 @@ StringTableAddString(HSTRING_TABLE hStringTable,
  *
  * RETURNS
  *     Success: String ID
- *     Failure: -1
+ *     Failure: ~0u
  *
  * NOTES
  *     If the given string already exists in the string table it will not
@@ -304,14 +212,99 @@ StringTableAddString(HSTRING_TABLE hStringTable,
  *     this case.
  */
 DWORD WINAPI
-StringTableAddStringEx(HSTRING_TABLE hStringTable,
-                       LPWSTR lpString,
-                       DWORD dwFlags,
-                       LPVOID lpExtraData,
-                       DWORD dwExtraDataSize)
+StringTableAddStringEx(HSTRING_TABLE hStringTable, LPWSTR lpString,
+                       DWORD dwFlags, LPVOID lpExtraData, DWORD dwExtraDataSize)
 {
-    FIXME("\n");
-    return (DWORD)-1;
+    PSTRING_TABLE pStringTable;
+    DWORD i;
+
+    TRACE("%p %s %x %p, %u\n", hStringTable, debugstr_w(lpString), dwFlags,
+          lpExtraData, dwExtraDataSize);
+
+    pStringTable = (PSTRING_TABLE)hStringTable;
+    if (!pStringTable)
+    {
+        ERR("Invalid hStringTable!\n");
+        return ~0u;
+    }
+
+    /* Search for existing string in the string table */
+    for (i = 0; i < pStringTable->dwMaxSlots; i++)
+    {
+        if (pStringTable->pSlots[i].pString)
+        {
+            if (dwFlags & 1)
+            {
+                if (!lstrcmpW(pStringTable->pSlots[i].pString, lpString))
+                    return i + 1;
+            }
+            else
+            {
+                if (!lstrcmpiW(pStringTable->pSlots[i].pString, lpString))
+                    return i + 1;
+            }
+        }
+    }
+
+    /* Check for filled slot table */
+    if (pStringTable->dwUsedSlots == pStringTable->dwMaxSlots)
+    {
+        FIXME("Resize the string table!\n");
+        return ~0u;
+    }
+
+    /* Search for an empty slot */
+    for (i = 0; i < pStringTable->dwMaxSlots; i++)
+    {
+        if (!pStringTable->pSlots[i].pString)
+        {
+            pStringTable->pSlots[i].pString = MyMalloc((lstrlenW(lpString) + 1) * sizeof(WCHAR));
+            if (!pStringTable->pSlots[i].pString)
+            {
+                WARN("Couldn't allocate memory for a new string!\n");
+                return ~0u;
+            }
+            lstrcpyW(pStringTable->pSlots[i].pString, lpString);
+
+            pStringTable->pSlots[i].pData = MyMalloc(dwExtraDataSize);
+            if (!pStringTable->pSlots[i].pData)
+            {
+                TRACE("Couldn't allocate memory for data!\n");
+                return ~0u;
+            }
+            memcpy(pStringTable->pSlots[i].pData, lpExtraData, dwExtraDataSize);
+            pStringTable->dwUsedSlots++;
+            return i + 1;
+        }
+    }
+    TRACE("Couldn't find an empty slot!\n");
+    return ~0u;
+}
+
+/**************************************************************************
+ * StringTableAddString [SETUPAPI.@]
+ *
+ * Adds a new string to the string table.
+ *
+ * PARAMS
+ *     hStringTable [I] Handle to the string table
+ *     lpString     [I] String to be added to the string table
+ *     dwFlags      [I] Flags
+ *                        1: case sensitive compare
+ *
+ * RETURNS
+ *     Success: String ID
+ *     Failure: ~0u
+ *
+ * NOTES
+ *     If the given string already exists in the string table it will not
+ *     be added again. The ID of the existing string will be returned in
+ *     this case.
+ */
+DWORD WINAPI
+StringTableAddString(HSTRING_TABLE hStringTable, LPWSTR lpString, DWORD dwFlags)
+{
+    return StringTableAddStringEx(hStringTable, lpString, dwFlags, NULL, 0);
 }