diff --git a/js/js_test.go b/js/js_test.go
index 0f308ef6e..2be9061c3 100644
--- a/js/js_test.go
+++ b/js/js_test.go
@@ -766,6 +766,7 @@ func TestJS(t *testing.T) {
 		{`var D;var{U,W,W}=y`, `var{U,W,W}=y,D`},
 		{`var A;var b=(function(){var e;})=c,d`, `var d,A,b=function(){var e}=c`},
 		{`0xB_BBBbAbA`, `3149642426`},
+		{`"\udFEb"`, `"\udFEb"`},
 
 		// bugs
 		{"var a=/\\s?auto?\\s?/i\nvar b;a,b", "var b,a=/\\s?auto?\\s?/i;a,b"},                           // #14
diff --git a/js/util.go b/js/util.go
index a5c287925..1844ef25d 100644
--- a/js/util.go
+++ b/js/util.go
@@ -1044,6 +1044,10 @@ func replaceEscapes(b []byte, quote byte, prefix, suffix int) []byte {
 					n += 2
 				}
 				m := utf8.RuneLen(rune(num))
+				if m == -1 {
+					i++
+					continue
+				}
 				utf8.EncodeRune(b[i+n-m:], rune(num))
 				n -= m
 			} else if c == '0' && (i+2 == len(b)-1 || b[i+2] < '0' || '7' < b[i+2]) {