--- a/intl/locale/mac/OSPreferences_mac.cpp
+++ b/intl/locale/mac/OSPreferences_mac.cpp
@@ -15,16 +15,17 @@ OSPreferences::ReadSystemLocales(nsTArra
   MOZ_ASSERT(aLocaleList.IsEmpty());
 
   // Get string representation of user's current locale
   CFLocaleRef userLocaleRef = ::CFLocaleCopyCurrent();
   CFStringRef userLocaleStr = ::CFLocaleGetIdentifier(userLocaleRef);
 
   AutoTArray<UniChar, 32> buffer;
   int size = ::CFStringGetLength(userLocaleStr);
+  buffer.SetLength(size);
 
   CFRange range = ::CFRangeMake(0, size);
   ::CFStringGetCharacters(userLocaleStr, range, buffer.Elements());
 
   // Convert the locale string to the format that Mozilla expects
   NS_LossyConvertUTF16toASCII locale(
       reinterpret_cast<const char16_t*>(buffer.Elements()), buffer.Length());