diff --git a/dlls/ntdll/rtlbitmap.c b/dlls/ntdll/rtlbitmap.c
index ef033b22848..806e0e80da5 100644
--- a/dlls/ntdll/rtlbitmap.c
+++ b/dlls/ntdll/rtlbitmap.c
@@ -555,9 +555,12 @@ ULONG WINAPI RtlNumberOfSetBits(PCRTL_BITMAP lpBits)
       lpOut++;
     }
 
-    bMasked = *lpOut & NTDLL_maskBits[ulRemainder];
-    ulSet += NTDLL_nibbleBitCount[bMasked >> 4];
-    ulSet += NTDLL_nibbleBitCount[bMasked & 0xf];
+    if (ulRemainder)
+    {
+      bMasked = *lpOut & NTDLL_maskBits[ulRemainder];
+      ulSet += NTDLL_nibbleBitCount[bMasked >> 4];
+      ulSet += NTDLL_nibbleBitCount[bMasked & 0xf];
+    }
   }
   return ulSet;
 }