2 files changed, 7 insertions, 14 deletions
diff --git a/src/unicode/letter.go b/src/unicode/letter.go
index 462daf88b9..b43cc66e7d 100644
--- a/src/unicode/letter.go
+++ b/src/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'
@@ -331,9 +332,11 @@ type foldPair struct {
 //
 //	SimpleFold('1') = '1'
 //
+//	SimpleFold(-2) = -2
+//
 func SimpleFold(r rune) rune {
-	if r < 0 {
-		panic("unicode: negative rune is disallowed")
+	if r < 0 || r > MaxRune {
+		return r
 	}
 
 	if int(r) < len(asciiFold) {
diff --git a/src/unicode/letter_test.go b/src/unicode/letter_test.go
index 15e4ade2a3..3fe72ff13d 100644
--- a/src/unicode/letter_test.go
+++ b/src/unicode/letter_test.go
@@ -432,19 +432,9 @@ func TestSimpleFold(t *testing.T) {
 			r = out
 		}
 	}
-}
-
-func TestSimpleFoldPanic(t *testing.T) {
-	got := func() (r interface{}) {
-		defer func() { r = recover() }()
-		SimpleFold(-1)
-		return nil
-	}()
-	want := "unicode: negative rune is disallowed"
 
-	s, _ := got.(string)
-	if s != want {
-		t.Errorf("SimpleFold(-1) should panic, got: %q, want: %q", got, want)
+	if r := SimpleFold(-42); r != -42 {
+		t.Errorf("SimpleFold(-42) = %v, want -42", r)
 	}
 }