diff --git a/src/archive/tar/writer_test.go b/src/archive/tar/writer_test.go
index da3fb89e65..48f35e5133 100644
--- a/src/archive/tar/writer_test.go
+++ b/src/archive/tar/writer_test.go
@@ -1252,7 +1252,7 @@ func TestFileWriter(t *testing.T) {
 
 	for i, v := range vectors {
 		var wantStr string
-		bb := new(bytes.Buffer)
+		bb := new(strings.Builder)
 		w := testNonEmptyWriter{bb}
 		var fw fileWriter
 		switch maker := v.maker.(type) {
diff --git a/src/archive/zip/zip_test.go b/src/archive/zip/zip_test.go
index ead9cd3aab..a4b952efcc 100644
--- a/src/archive/zip/zip_test.go
+++ b/src/archive/zip/zip_test.go
@@ -24,7 +24,7 @@ func TestOver65kFiles(t *testing.T) {
 	if testing.Short() && testenv.Builder() == "" {
 		t.Skip("skipping in short mode")
 	}
-	buf := new(bytes.Buffer)
+	buf := new(strings.Builder)
 	w := NewWriter(buf)
 	const nFiles = (1 << 16) + 42
 	for i := 0; i < nFiles; i++ {