diff --git a/src/pkg/bytes/bytes.go b/src/pkg/bytes/bytes.go
index 31cf89ea87c..6d40af4436c 100644
--- a/src/pkg/bytes/bytes.go
+++ b/src/pkg/bytes/bytes.go
@@ -571,7 +571,10 @@ func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
 // inverted.
 func lastIndexFunc(s []byte, f func(r rune) bool, truth bool) int {
 	for i := len(s); i > 0; {
-		r, size := utf8.DecodeLastRune(s[0:i])
+		r, size := rune(s[i-1]), 1
+		if r >= utf8.RuneSelf {
+			r, size = utf8.DecodeLastRune(s[0:i])
+		}
 		i -= size
 		if f(r) == truth {
 			return i
diff --git a/src/pkg/bytes/bytes_test.go b/src/pkg/bytes/bytes_test.go
index 1d073b143bd..1d6274c33d1 100644
--- a/src/pkg/bytes/bytes_test.go
+++ b/src/pkg/bytes/bytes_test.go
@@ -1073,3 +1073,10 @@ func BenchmarkFieldsFunc(b *testing.B) {
 		FieldsFunc(fieldsInput, unicode.IsSpace)
 	}
 }
+
+func BenchmarkTrimSpace(b *testing.B) {
+	s := []byte("  Some text.  \n")
+	for i := 0; i < b.N; i++ {
+		TrimSpace(s)
+	}
+}