diff --git a/github/files_test.go b/github/files_test.go
index 51a970c..d45670d 100644
--- a/github/files_test.go
+++ b/github/files_test.go
@@ -110,7 +110,9 @@ func TestGetFileContent_429Retry(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	content, err := c.GetFileContent(context.Background(), "owner", "repo", "file.go", "")
 	if err != nil {
@@ -228,7 +230,9 @@ func TestListContents_429Retry(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	entries, err := c.ListContents(context.Background(), "owner", "repo", "src")
 	if err != nil {