diff --git a/string.c b/string.c
index c140148..3253e9c 100644
--- a/string.c
+++ b/string.c
@@ -3206,15 +3206,20 @@ static VALUE str_casecmp_p(VALUE str1, VALUE str2);
  *     "abcdef".casecmp("ABCDEF")    #=> 0
  *
  *  +nil+ is returned if the two strings have incompatible encodings.
+ *  or if +other_str+ is not a string and does not respond to +to_str+.
  *
  *     "\u{e4 f6 fc}".encode("ISO-8859-1").casecmp("\u{c4 d6 dc}")   #=> nil
+ *     "abcdef".casecmp(1)   #=> nil
  */
 
 static VALUE
 rb_str_casecmp(VALUE str1, VALUE str2)
 {
-    StringValue(str2);
-    return str_casecmp(str1, str2);
+    VALUE s = rb_check_string_type(str2);
+    if (NIL_P(s)) {
+      return Qnil;
+    }
+    return str_casecmp(str1, s);
 }
 
 static VALUE
@@ -3224,6 +3229,10 @@ str_casecmp(VALUE str1, VALUE str2)
     rb_encoding *enc;
     char *p1, *p1end, *p2, *p2end;
 
+    if (!rb_obj_is_kind_of(str2, rb_cString)) {
+      return Qnil;
+    }
+
     enc = rb_enc_compatible(str1, str2);
     if (!enc) {
 	return Qnil;
@@ -3288,15 +3297,21 @@ str_casecmp(VALUE str1, VALUE str2)
  *     "\u{e4 f6 fc}".casecmp?("\u{c4 d6 dc}")   #=> true
  *
  *  +nil+ is returned if the two strings have incompatible encodings.
+ *  or if +other_str+ is not a string and does not respond to +to_str+.
  *
  *     "\u{e4 f6 fc}".encode("ISO-8859-1").casecmp?("\u{c4 d6 dc}")   #=> nil
+ *     "abcdef".casecmp?(1)   #=> nil
  */
 
 static VALUE
+
 rb_str_casecmp_p(VALUE str1, VALUE str2)
 {
-    StringValue(str2);
-    return str_casecmp_p(str1, str2);
+    VALUE s = rb_check_string_type(str2);
+    if (NIL_P(s)) {
+      return Qnil;
+    }
+    return str_casecmp_p(str1, s);
 }
 
 static VALUE
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 7dbf27e..3b72d25 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -2318,6 +2318,12 @@ def test_compare_different_encoding_string
 
   def test_casecmp
     assert_equal(1, "\u3042B".casecmp("\u3042a"))
+    assert_nil("Foo".casecmp(:Foo))
+    assert_nil("1".casecmp(1))
+
+    o = Object.new
+    def o.to_str; "abcde"; end
+    assert_equal(1, "abcdef".casecmp(o))
   end
 
   def test_casecmp?
@@ -2325,6 +2331,12 @@ def test_casecmp?
     assert_equal(false, 'FoO'.casecmp?('BaR'))
     assert_equal(false, 'baR'.casecmp?('FoO'))
     assert_equal(true, 'äöü'.casecmp?('ÄÖÜ'))
+    assert_nil("Foo".casecmp?(:Foo))
+    assert_nil("1".casecmp?(1))
+
+    o = Object.new
+    def o.to_str; "foo"; end
+    assert_equal(true, "Foo".casecmp?(o))
   end
 
   def test_upcase2