--- a/js/src/builtin/Intl.cpp
+++ b/js/src/builtin/Intl.cpp
@@ -1124,16 +1124,22 @@ js::intl_availableCollations(JSContext* 
         return false;
     }
 
     RootedObject collations(cx, NewDenseEmptyArray(cx));
     if (!collations)
         return false;
 
     uint32_t index = 0;
+
+    // The first element of the collations array must be |null| per
+    // ES2017 Intl, 10.2.3 Internal Slots.
+    if (!DefineElement(cx, collations, index++, NullHandleValue))
+        return false;
+
     for (uint32_t i = 0; i < count; i++) {
         const char* collation = uenum_next(values, nullptr, &status);
         if (U_FAILURE(status)) {
             JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_INTERNAL_INTL_ERROR);
             return false;
         }
 
         // Per ECMA-402, 10.2.3, we don't include standard and search:

--- a/js/src/builtin/Intl.js
+++ b/js/src/builtin/Intl.js
@@ -1601,20 +1601,18 @@ var collatorInternalProperties = {
         addSpecialMissingLanguageTags(locales);
         return (this._availableLocales = locales);
     },
     relevantExtensionKeys: ["co", "kn"]
 };
 
 
 function collatorSortLocaleData(locale) {
-    var collations = intl_availableCollations(locale);
-    callFunction(std_Array_unshift, collations, null);
     return {
-        co: collations,
+        co: intl_availableCollations(locale),
         kn: ["false", "true"]
     };
 }
 
 
 function collatorSearchLocaleData(locale) {
     return {
         co: [null],