diff --git a/mrbgems/mruby-rational/src/rational.c b/mrbgems/mruby-rational/src/rational.c
index d15af73ce..9b2651c33 100644
--- a/mrbgems/mruby-rational/src/rational.c
+++ b/mrbgems/mruby-rational/src/rational.c
@@ -311,7 +311,7 @@ int_lshift(mrb_state *mrb, mrb_value v, mrb_int n)
 {
   if (mrb_integer_p(v)) {
     mrb_float f = (mrb_float)mrb_integer(v);
-    f *= 1<<n;
+    f *= 1L<<n;
     if (mrb_int_fit_p(f, mrb_float))
       return mrb_int_value(mrb, (mrb_int)f);
   }