fix potential nil-pointer dereference

the fix gracefully fails the assertion instead of panicking unresolved.

assert/assertions.go

@@ -721,7 +721,11 @@ func NotEqualValues(t TestingT, expected, actual interface{}, msgAndArgs ...inte
 func includeElement(list interface{}, element interface{}) (ok, found bool) {
 
 	listValue := reflect.ValueOf(list)
-	listKind := reflect.TypeOf(list).Kind()
+	listType := reflect.TypeOf(list)
+	if listType == nil {
+		return false, false
+	}
+	listKind := listType.Kind()
 	defer func() {
 		if e := recover(); e != nil {
 			ok = false

assert/assertions_test.go

@@ -592,6 +592,7 @@ func TestContainsNotContains(t *testing.T) {
 		{"j", "k"},
 	}
 	simpleMap := map[interface{}]interface{}{"Foo": "Bar"}
+	var zeroMap map[interface{}]interface{}
 
 	cases := []struct {
 		expected interface{}
@@ -606,6 +607,7 @@ func TestContainsNotContains(t *testing.T) {
 		{complexList, &A{"g", "e"}, false},
 		{simpleMap, "Foo", true},
 		{simpleMap, "Bar", false},
+		{zeroMap, "Bar", false},
 	}
 
 	for _, c := range cases {
@@ -652,6 +654,22 @@ func TestContainsFailMessage(t *testing.T) {
 	}
 }
 
+func TestContainsNotContainsOnNilValue(t *testing.T) {
+	mockT := new(mockTestingT)
+
+	Contains(mockT, nil, "key")
+	expectedFail := "<nil> could not be applied builtin len()"
+	actualFail := mockT.errorString()
+	if !strings.Contains(actualFail, expectedFail) {
+		t.Errorf("Contains failure should include %q but was %q", expectedFail, actualFail)
+	}
+
+	NotContains(mockT, nil, "key")
+	if !strings.Contains(actualFail, expectedFail) {
+		t.Errorf("Contains failure should include %q but was %q", expectedFail, actualFail)
+	}
+}
+
 func TestSubsetNotSubset(t *testing.T) {
 
 	// MTestCase adds a custom message to the case