diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index 45f19a95c..5d6401e3a 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -608,7 +608,7 @@ retry:
               }
               mrb_value str = mrb_bint_to_s(mrb, val, base);
               s = RSTRING_PTR(str);
-              len = RSTRING_LEN(str);
+              len = (int)RSTRING_LEN(str);
             }
             goto str_skip;
 #endif