--- a/js/src/vm/JSContext.cpp
+++ b/js/src/vm/JSContext.cpp
@@ -879,19 +879,19 @@ js::ReportErrorNumberUCArray(JSContext* 
 
     return warning;
 }
 
 void
 js::ReportIsNotDefined(JSContext* cx, HandleId id)
 {
     JSAutoByteString printable;
-    if (!ValueToPrintableLatin1(cx, IdToValue(id), &printable))
+    if (!ValueToPrintableUTF8(cx, IdToValue(id), &printable))
         return;
-    JS_ReportErrorNumberLatin1(cx, GetErrorMessage, nullptr, JSMSG_NOT_DEFINED, printable.ptr());
+    JS_ReportErrorNumberUTF8(cx, GetErrorMessage, nullptr, JSMSG_NOT_DEFINED, printable.ptr());
 }
 
 void
 js::ReportIsNotDefined(JSContext* cx, HandlePropertyName name)
 {
     RootedId id(cx, NameToId(name));
     ReportIsNotDefined(cx, id);
 }

--- a/js/src/vm/StringType.cpp
+++ b/js/src/vm/StringType.cpp
@@ -1921,16 +1921,30 @@ js::ValueToPrintableLatin1(JSContext* cx
     if (!str)
         return nullptr;
     str = QuoteString(cx, str, 0);
     if (!str)
         return nullptr;
     return bytes->encodeLatin1(cx, str);
 }
 
+const char*
+js::ValueToPrintableUTF8(JSContext* cx, const Value& vArg, JSAutoByteString* bytes, bool asSource)
+{
+    RootedValue v(cx, vArg);
+    JSString* str;
+    if (asSource)
+        str = ValueToSource(cx, v);
+    else
+        str = ToString<CanGC>(cx, v);
+    if (!str)
+        return nullptr;
+    return bytes->encodeUtf8(cx, RootedString(cx, str));
+}
+
 template <AllowGC allowGC>
 JSString*
 js::ToStringSlow(JSContext* cx, typename MaybeRooted<Value, allowGC>::HandleType arg)
 {
     /* As with ToObjectSlow, callers must verify that |arg| isn't a string. */
     MOZ_ASSERT(!arg.isString());
 
     Value v = arg;

--- a/js/src/vm/StringType.h
+++ b/js/src/vm/StringType.h
@@ -1594,16 +1594,22 @@ SubstringKernel(JSContext* cx, HandleStr
  * will be changed to Unicode escape sequences(I.e. \udddd, dddd are 4 hex digits) in the printable
  * string.
  */
 extern const char*
 ValueToPrintableLatin1(JSContext* cx, const Value&, JSAutoByteString* bytes,
                        bool asSource = false);
 
 /*
+ * Convert a value to a printable C string encoded in UTF-8.
+ */
+extern const char*
+ValueToPrintableUTF8(JSContext* cx, const Value&, JSAutoByteString* bytes, bool asSource = false);
+
+/*
  * Convert a non-string value to a string, returning null after reporting an
  * error, otherwise returning a new string reference.
  */
 template <AllowGC allowGC>
 extern JSString*
 ToStringSlow(JSContext* cx, typename MaybeRooted<Value, allowGC>::HandleType arg);
 
 /*