diff --git a/ext/opencv/cvmat.cpp b/ext/opencv/cvmat.cpp
index d234099..5a6d1af 100644
--- a/ext/opencv/cvmat.cpp
+++ b/ext/opencv/cvmat.cpp
@@ -1546,16 +1546,19 @@ VALUE
 rb_flip_bang(int argc, VALUE *argv, VALUE self)
 {
   VALUE format;
-  int mode = 0;
+  int mode = 1;
   if (rb_scan_args(argc, argv, "01", &format) > 0) {
-    if (rb_to_id(format) == rb_intern("x"))
+    Check_Type(format, T_SYMBOL);
+    ID flip_mode = rb_to_id(format);
+    if (flip_mode == rb_intern("x")) {
       mode = 1;
-    else if (rb_to_id(format) == rb_intern("y"))
+    }
+    else if (flip_mode == rb_intern("y")) {
       mode = 0;
-    else if (rb_to_id(format) == rb_intern("xy"))
+    }
+    else if (flip_mode == rb_intern("xy")) {
       mode = -1;
-    else
-      rb_warn("argument may be :x or :y or :xy");
+    }
   }
   try {
     cvFlip(CVARR(self), NULL, mode);
diff --git a/test/test_cvmat.rb b/test/test_cvmat.rb
index 017fe1b..c71fa65 100755
--- a/test/test_cvmat.rb
+++ b/test/test_cvmat.rb
@@ -809,10 +809,17 @@ class TestCvMat < OpenCVTestCase
         assert_cvscalar_equal(m0[rj, i], m4[j, i])
         assert_cvscalar_equal(m0[rj, ri], m5[j, i])
         assert_cvscalar_equal(m0[rj, ri], m6[j, i])
-        assert_cvscalar_equal(m0[rj, i], m7[j, i])
-        assert_cvscalar_equal(m0[rj, i], m8[j, i])
+        assert_cvscalar_equal(m0[j, ri], m7[j, i])
+        assert_cvscalar_equal(m0[j, ri], m8[j, i])
       }
     }
+
+    assert_raise(TypeError) {
+      m0.flip(DUMMY_OBJ)
+    }
+    assert_raise(TypeError) {
+      m0.flip!(DUMMY_OBJ)
+    }
   end
 
   def test_split