diff --git a/digitalclock/main_test.go b/digitalclock/main_test.go
index dcb2d79..cf10f7f 100644
--- a/digitalclock/main_test.go
+++ b/digitalclock/main_test.go
@@ -115,7 +115,7 @@ func queryImage(t *testing.T, c *http.Client, url string) image.Image {
 	require.Equal(t, "image/png", resp.Header.Get("Content-Type"))
 
 	img, err := png.Decode(resp.Body)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	return img
 }
@@ -125,7 +125,7 @@ func TestDigitalClock_valid(t *testing.T) {
 	defer stop()
 
 	files, err := ioutil.ReadDir("./testdata")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	c := &http.Client{Timeout: time.Second * 10}
 
@@ -171,7 +171,7 @@ func TestDigitalClock_invalid(t *testing.T) {
 
 	testName := func(tc *TestCase) string {
 		data, err := json.Marshal(tc)
-		require.Nil(t, err)
+		require.NoError(t, err)
 		return string(data)
 	}
 
diff --git a/tools/testtool/freeport_test.go b/tools/testtool/freeport_test.go
index 2189fb7..3c9e584 100644
--- a/tools/testtool/freeport_test.go
+++ b/tools/testtool/freeport_test.go
@@ -22,9 +22,9 @@ func TestWaitForPort(t *testing.T) {
 	defer s.Close()
 
 	u, err := url.Parse(s.URL)
-	require.Nil(t, err)
+	require.NoError(t, err)
 	_, port, err := net.SplitHostPort(u.Host)
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	require.NoError(t, WaitForPort(t, time.Second, port))
 }
diff --git a/tools/testtool/testdata/submissions/correct/sum/private/sum/sum_private_test.go b/tools/testtool/testdata/submissions/correct/sum/private/sum/sum_private_test.go
index 51d8825..526f814 100644
--- a/tools/testtool/testdata/submissions/correct/sum/private/sum/sum_private_test.go
+++ b/tools/testtool/testdata/submissions/correct/sum/private/sum/sum_private_test.go
@@ -36,7 +36,7 @@ func readTestCases(filename string) ([]*testCase, error) {
 
 func TestSumPrivate(t *testing.T) {
 	tests, err := readTestCases("./testdata/tests.csv")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	for _, tc := range tests {
 		s := Sum(tc.a, tc.b)
diff --git a/tools/testtool/testdata/submissions/incorrect/brokenstyle/private/brokenstyle/sum_private_test.go b/tools/testtool/testdata/submissions/incorrect/brokenstyle/private/brokenstyle/sum_private_test.go
index d51f782..0fd61be 100644
--- a/tools/testtool/testdata/submissions/incorrect/brokenstyle/private/brokenstyle/sum_private_test.go
+++ b/tools/testtool/testdata/submissions/incorrect/brokenstyle/private/brokenstyle/sum_private_test.go
@@ -36,7 +36,7 @@ func readTestCases(filename string) ([]*testCase, error) {
 
 func TestSumPrivate(t *testing.T) {
 	tests, err := readTestCases("./testdata/tests.csv")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	for _, tc := range tests {
 		s := Sum(tc.a, tc.b)
diff --git a/tools/testtool/testdata/submissions/incorrect/brokentest/private/brokentest/sum_private_test.go b/tools/testtool/testdata/submissions/incorrect/brokentest/private/brokentest/sum_private_test.go
index 7878f07..d3d265d 100644
--- a/tools/testtool/testdata/submissions/incorrect/brokentest/private/brokentest/sum_private_test.go
+++ b/tools/testtool/testdata/submissions/incorrect/brokentest/private/brokentest/sum_private_test.go
@@ -36,7 +36,7 @@ func readTestCases(filename string) ([]*testCase, error) {
 
 func TestSumPrivate(t *testing.T) {
 	tests, err := readTestCases("./testdata/tests.csv")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	for _, tc := range tests {
 		s := Sum(tc.a, tc.b)
diff --git a/tools/testtool/testdata/submissions/incorrect/newdependency/private/newdependency/sum_private_test.go b/tools/testtool/testdata/submissions/incorrect/newdependency/private/newdependency/sum_private_test.go
index e9b2d9a..ec91678 100644
--- a/tools/testtool/testdata/submissions/incorrect/newdependency/private/newdependency/sum_private_test.go
+++ b/tools/testtool/testdata/submissions/incorrect/newdependency/private/newdependency/sum_private_test.go
@@ -36,7 +36,7 @@ func readTestCases(filename string) ([]*testCase, error) {
 
 func TestSumPrivate(t *testing.T) {
 	tests, err := readTestCases("./testdata/tests.csv")
-	require.Nil(t, err)
+	require.NoError(t, err)
 
 	for _, tc := range tests {
 		s := Sum(tc.a, tc.b)