--- a/src/chop.c
+++ b/src/chop.c
@@ -539,7 +539,7 @@ extern const chop_class_t chop_gdbm_bloc
   chop_qdbm_block_iterator_class;
 
 const struct chop_class_entry *
-chop_lookup_class_entry (const char *str, unsigned int len);
+chop_lookup_class_entry (const char *str, size_t len);
 
 /* Include the gperf-generated perfect hash table.  */
 #include "class-lookup.c"