diff --git a/errors_test.go b/errors_test.go
new file mode 100644
index 0000000..289bff7
--- /dev/null
+++ b/errors_test.go
@@ -0,0 +1,32 @@
+package stun
+
+import "testing"
+
+func TestDecodeErr_IsInvalidCookie(t *testing.T) {
+	m := new(Message)
+	m.WriteHeader()
+	decoded := new(Message)
+	m.Raw[4] = 55
+	_, err := decoded.Write(m.Raw)
+	if err == nil {
+		t.Fatal("should error")
+	}
+	expected := "BadFormat for message/cookie: " +
+		"3712a442 is invalid magic cookie (should be 2112a442)"
+	if err.Error() != expected {
+		t.Error(err, "!=", expected)
+	}
+	dErr, ok := err.(*DecodeErr)
+	if !ok {
+		t.Error("not decode error")
+	}
+	if !dErr.IsInvalidCookie() {
+		t.Error("IsInvalidCookie = false, should be true")
+	}
+	if !dErr.IsPlaceChildren("cookie") {
+		t.Error("bad children")
+	}
+	if !dErr.IsPlaceParent("message") {
+		t.Error("bad parent")
+	}
+}