diff --git a/dlls/ntdll/rtlbitmap.c b/dlls/ntdll/rtlbitmap.c
index d04754a39dc..d134025239b 100644
--- a/dlls/ntdll/rtlbitmap.c
+++ b/dlls/ntdll/rtlbitmap.c
@@ -592,9 +592,9 @@ CCHAR WINAPI RtlFindMostSignificantBit(ULONGLONG ulLong)
   {
     if (*lpOut)
     {
-      if (*lpOut & 0x0f)
-        return lCount - 8 + NTDLL_mostSignificant[*lpOut & 0x0f];
-      return lCount - 4 + NTDLL_mostSignificant[*lpOut >> 4];
+      if (*lpOut & 0xf0)
+        return lCount - 4 + NTDLL_mostSignificant[*lpOut >> 4];
+      return lCount - 8 + NTDLL_mostSignificant[*lpOut & 0x0f];
     }
     lpOut--;
   }
diff --git a/dlls/ntdll/tests/rtlbitmap.c b/dlls/ntdll/tests/rtlbitmap.c
index 0ff0b50d462..f506be17f85 100644
--- a/dlls/ntdll/tests/rtlbitmap.c
+++ b/dlls/ntdll/tests/rtlbitmap.c
@@ -444,7 +444,13 @@ static void test_RtlFindMostSignificantBit()
     ulLong <<= i;
 
     cPos = pRtlFindMostSignificantBit(ulLong);
-    ok (cPos == i, "didnt find MSB %lld %d %d", ulLong, i, cPos);
+    ok (cPos == i, "didnt find MSB %llx %d %d", ulLong, i, cPos);
+
+    /* Set all bits lower than bit i */
+    ulLong = ((ulLong - 1) << 1) | 1;
+
+    cPos = pRtlFindMostSignificantBit(ulLong);
+    ok (cPos == i, "didnt find MSB %llx %d %d", ulLong, i, cPos);
   }
   cPos = pRtlFindMostSignificantBit(0);
   ok (cPos == -1, "found bit when not set");
@@ -465,7 +471,13 @@ static void test_RtlFindLeastSignificantBit()
     ulLong <<= i;
 
     cPos = pRtlFindLeastSignificantBit(ulLong);
-    ok (cPos == i, "didnt find LSB at %d", cPos);
+    ok (cPos == i, "didnt find LSB %llx %d %d", ulLong, i, cPos);
+
+    ulLong = 0xfffffffffffffffful;
+    ulLong <<= i;
+
+    cPos = pRtlFindLeastSignificantBit(ulLong);
+    ok (cPos == i, "didnt find LSB %llx %d %d", ulLong, i, cPos);
   }
   cPos = pRtlFindLeastSignificantBit(0);
   ok (cPos == -1, "found bit when not set");