diff --git a/reactos/lib/rtl/bitmap.c b/reactos/lib/rtl/bitmap.c
index 6ceddf927ee..350e182191c 100644
--- a/reactos/lib/rtl/bitmap.c
+++ b/reactos/lib/rtl/bitmap.c
@@ -101,6 +101,11 @@ RtlpGetLengthOfRunClear(
     BITMAP_INDEX Value, BitPos, Length;
     PBITMAP_BUFFER Buffer, MaxBuffer;
 
+    /* If we are already at the end, the length of the run is zero */
+    ASSERT(StartingIndex <= BitMapHeader->SizeOfBitMap);
+    if (StartingIndex >= BitMapHeader->SizeOfBitMap)
+        return 0;
+
     /* Calculate positions */
     Buffer = BitMapHeader->Buffer + StartingIndex / _BITCOUNT;
     BitPos = StartingIndex & (_BITCOUNT - 1);
@@ -150,6 +155,11 @@ RtlpGetLengthOfRunSet(
     BITMAP_INDEX InvValue, BitPos, Length;
     PBITMAP_BUFFER Buffer, MaxBuffer;
 
+    /* If we are already at the end, the length of the run is zero */
+    ASSERT(StartingIndex <= BitMapHeader->SizeOfBitMap);
+    if (StartingIndex >= BitMapHeader->SizeOfBitMap)
+        return 0;
+
     /* Calculate positions */
     Buffer = BitMapHeader->Buffer + StartingIndex / _BITCOUNT;
     BitPos = StartingIndex & (_BITCOUNT - 1);