diff --git a/include/ruby/intern.h b/include/ruby/intern.h
index dd259e8fe2..2d29f44fc8 100644
--- a/include/ruby/intern.h
+++ b/include/ruby/intern.h
@@ -515,7 +515,7 @@ VALUE rb_str_unlocktmp(VALUE);
 VALUE rb_str_dup_frozen(VALUE);
 VALUE rb_str_plus(VALUE, VALUE);
 VALUE rb_str_times(VALUE, VALUE);
-int rb_str_sublen(VALUE, int);
+long rb_str_sublen(VALUE, long);
 VALUE rb_str_substr(VALUE, long, long);
 VALUE rb_str_subseq(VALUE, long, long);
 void rb_str_modify(VALUE);