diff --git a/src/strings/strings.go b/src/strings/strings.go
index 00200e4e24d..ecc8c97d9eb 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -463,27 +463,27 @@ func Map(mapping func(rune) rune, s string) string {
 
 	for i, c := range s {
 		r := mapping(c)
-		if r == c {
+		if r == c && c != utf8.RuneError {
 			continue
 		}
 
+		var width int
+		if c == utf8.RuneError {
+			c, width = utf8.DecodeRuneInString(s[i:])
+			if width != 1 && r == c {
+				continue
+			}
+		} else {
+			width = utf8.RuneLen(c)
+		}
+
 		b.Grow(len(s) + utf8.UTFMax)
 		b.WriteString(s[:i])
 		if r >= 0 {
 			b.WriteRune(r)
 		}
 
-		if c == utf8.RuneError {
-			// RuneError is the result of either decoding
-			// an invalid sequence or '\uFFFD'. Determine
-			// the correct number of bytes we need to advance.
-			_, w := utf8.DecodeRuneInString(s[i:])
-			i += w
-		} else {
-			i += utf8.RuneLen(c)
-		}
-
-		s = s[i:]
+		s = s[i+width:]
 		break
 	}
 
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index bb6a5b931b8..eee2dd55dfd 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -646,10 +646,10 @@ func TestMap(t *testing.T) {
 		if unicode.Is(unicode.Latin, r) {
 			return r
 		}
-		return '?'
+		return utf8.RuneError
 	}
 	m = Map(replaceNotLatin, "Hello\255World")
-	expect = "Hello?World"
+	expect = "Hello\uFFFDWorld"
 	if m != expect {
 		t.Errorf("replace invalid sequence: expected %q got %q", expect, m)
 	}