diff --git a/nimPNG.nim b/nimPNG.nim
index 166474f..a51c856 100644
--- a/nimPNG.nim
+++ b/nimPNG.nim
@@ -226,8 +226,8 @@ proc crc32(crc: uint32, buf: string): uint32 =
 
   var crcu32 = not crc
   for b in buf:
-    crcu32 = (crcu32 shr 4) xor kcrc32[(crcu32 and 0xF) xor (uint32(b) and 0xF'u32)]
-    crcu32 = (crcu32 shr 4) xor kcrc32[(crcu32 and 0xF) xor (uint32(b) shr 4'u32)]
+    crcu32 = (crcu32 shr 4) xor kcrc32[int((crcu32 and 0xF) xor (uint32(b) and 0xF'u32))]
+    crcu32 = (crcu32 shr 4) xor kcrc32[int((crcu32 and 0xF) xor (uint32(b) shr 4'u32))]
 
   result = not crcu32