diff --git a/sprintf.c b/sprintf.c
index f7ebe82e2c..22323265b3 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -250,6 +250,7 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
     --argv;
     StringValue(fmt);
     enc = rb_enc_get(fmt);
+    rb_must_asciicompat(fmt);
     orig = fmt;
     fmt = rb_str_tmp_frozen_acquire(fmt);
     p = RSTRING_PTR(fmt);
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index a50507a528..da04ae7fa7 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -898,6 +898,16 @@ class TestM17N < Test::Unit::TestCase
 
   def test_sprintf_p
     Encoding.list.each do |e|
+      unless e.ascii_compatible?
+        format = e.dummy? ? "%p".force_encoding(e) : "%p".encode(e)
+        assert_raise(Encoding::CompatibilityError) do
+          sprintf(format, nil)
+        end
+        assert_raise(Encoding::CompatibilityError) do
+          format % nil
+        end
+        next
+      end
       format = "%p".force_encoding(e)
       ['', 'a', "\xC2\xA1", "\x00"].each do |s|
         s.force_encoding(e)