diff --git a/lib/checksum.h b/lib/checksum.h
index d3ce930..da1d3cb 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1,2 +1,2 @@
 extern int in_cksum(void *, int);
-extern u_int16_t fletcher_checksum(u_char * buffer, int len, u_int16_t offset);
+extern u_int16_t fletcher_checksum(u_char *, const size_t len, const uint16_t offset);