diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index 75086f1f24..d7b34bd0d8 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -146,7 +146,7 @@ func TestReader(t *testing.T) {
 	for i := 0; i < len(texts)-1; i++ {
 		texts[i] = str + "\n"
 		all += texts[i]
-		str += string(rune(i)%26 + 'a')
+		str += string(rune(i%26 + 'a'))
 	}
 	texts[len(texts)-1] = all