diff --git a/include/mimalloc/internal.h b/include/mimalloc/internal.h
index 4cac7a88..26916616 100644
--- a/include/mimalloc/internal.h
+++ b/include/mimalloc/internal.h
@@ -938,7 +938,7 @@ static inline size_t mi_clz(size_t x) {
   #else
     _BitScanReverse64(&idx, x);
   #endif
-  return ((MI_SIZE_BITS - 1) - idx);
+  return ((MI_SIZE_BITS - 1) - (size_t)idx);
 }
 static inline size_t mi_ctz(size_t x) {
   if (x==0) return MI_SIZE_BITS;
@@ -948,7 +948,7 @@ static inline size_t mi_ctz(size_t x) {
   #else
     _BitScanForward64(&idx, x);
   #endif
-  return idx;
+  return (size_t)idx;
 }
 
 #else