1 files changed, 7 insertions, 0 deletions
diff --git a/libgo/go/unicode/letter.go b/libgo/go/unicode/letter.go
index 8aec920d22f..b43cc66e7d3 100644
--- a/libgo/go/unicode/letter.go
+++ b/libgo/go/unicode/letter.go
@@ -320,6 +320,7 @@ type foldPair struct {
 // the Unicode-defined simple case folding. Among the code points
 // equivalent to rune (including rune itself), SimpleFold returns the
 // smallest rune > r if one exists, or else the smallest rune >= 0.
+// If r is not a valid Unicode code point, SimpleFold(r) returns r.
 //
 // For example:
 //	SimpleFold('A') = 'a'
@@ -331,7 +332,13 @@ type foldPair struct {
 //
 //	SimpleFold('1') = '1'
 //
+//	SimpleFold(-2) = -2
+//
 func SimpleFold(r rune) rune {
+	if r < 0 || r > MaxRune {
+		return r
+	}
+
 	if int(r) < len(asciiFold) {
 		return rune(asciiFold[r])
 	}