diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index 168b3d0..70f422a 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -395,7 +395,7 @@ checksum_long(uLong (*func)(uLong, const Bytef*, uInt), uLong sum, const Bytef *
     return sum;
 }
 #else
-#define checksum_long(func, sum, ptr, len) (func)((sum), (ptr), (len))
+#define checksum_long(func, sum, ptr, len) (func)(mask32(sum), (ptr), (len))
 #endif
 
 static VALUE