diff --git a/mrbgems/mruby-rational/src/rational.c b/mrbgems/mruby-rational/src/rational.c
index 6c9b238af..70b8cc451 100644
--- a/mrbgems/mruby-rational/src/rational.c
+++ b/mrbgems/mruby-rational/src/rational.c
@@ -722,7 +722,7 @@ rational_pow(mrb_state *mrb, mrb_value x)
     }
     break;
   default:
-    mrb_raisef(mrb, E_TYPE_ERROR, "%T cannot convert to Rational", y);
+    mrb_raisef(mrb, E_TYPE_ERROR, "%T cannot be converted to Rational", y);
   }
   return mrb_float_value(mrb, pow(d1, d2));
 #else