diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index f49c3d6f8..eec001264 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -607,7 +607,10 @@ mrb_str_format(mrb_state *mrb, mrb_int argc, const mrb_value *argv, mrb_value fm
 
     for (t = p; t < end && *t != '%'; t++)
       ;
-    if (t + 1 == end) t++;
+    if (t + 1 == end) {
+      /* % at the bottom */
+      mrb_raise(mrb, E_ARGUMENT_ERROR, "incomplete format specifier; use %% (double %) instead");
+    }
     PUSH(p, t - p);
     if (t >= end)
       goto sprint_exit; /* end of fmt string */
diff --git a/mrbgems/mruby-sprintf/test/sprintf.rb b/mrbgems/mruby-sprintf/test/sprintf.rb
index aedc2787a..1b1fe95f1 100644
--- a/mrbgems/mruby-sprintf/test/sprintf.rb
+++ b/mrbgems/mruby-sprintf/test/sprintf.rb
@@ -90,8 +90,3 @@ assert("String#% invalid format") do
     "%?" % ""
   end
 end
-
-assert("String#% invalid format shared substring") do
-  fmt = ("x"*30+"%!")[0...-1]
-  assert_equal fmt, sprintf(fmt, "")
-end