diff --git a/src/hammer.h b/src/hammer.h
index 95da0eeda926d456ed3cba061a5b7571b4e99c14..62b5676610415b0a03a9546cea7b829b3477d658 100644
--- a/src/hammer.h
+++ b/src/hammer.h
@@ -229,7 +229,7 @@ const HParser* h_action(const HParser* p, const HAction a);
  *
  * Result token type: TT_UINT
  */
-const HParser* h_not_in(const uint8_t *charset, int length);
+const HParser* h_not_in(const uint8_t *charset, size_t length);
 
 /**
  * A no-argument parser that succeeds if there is no more input to