diff --git a/numeric.c b/numeric.c
index 2471f9bd04..c880199148 100644
--- a/numeric.c
+++ b/numeric.c
@@ -233,6 +233,7 @@ rb_num_get_rounding_option(VALUE opts)
 	    str = rb_check_string_type(rounding);
 	    if (NIL_P(str)) goto invalid;
 	}
+        rb_must_asciicompat(str);
 	s = RSTRING_PTR(str);
 	switch (RSTRING_LEN(str)) {
 	  case 2:
diff --git a/test/ruby/test_float.rb b/test/ruby/test_float.rb
index 7cbf3b5a8f..a94bbce49e 100644
--- a/test/ruby/test_float.rb
+++ b/test/ruby/test_float.rb
@@ -764,6 +764,9 @@ class TestFloat < Test::Unit::TestCase
     assert_raise_with_message(ArgumentError, /xxx/) {
       1.0.round(half: "\0xxx")
     }
+    assert_raise_with_message(Encoding::CompatibilityError, /ASCII incompatible/) {
+      1.0.round(half: "up".force_encoding("utf-16be"))
+    }
   end
 
   def test_Float