diff --git a/mfbt/RangedPtr.h b/mfbt/RangedPtr.h
index 6e9ce6072..df398c112 100644
--- a/mfbt/RangedPtr.h
+++ b/mfbt/RangedPtr.h
@@ -212,7 +212,7 @@ public:
     return *this;
   }
 
-  T& operator[](int aIndex) const
+  T& operator[](ptrdiff_t aIndex) const
   {
     MOZ_ASSERT(size_t(aIndex > 0 ? aIndex : -aIndex) <= size_t(-1) / sizeof(T));
     return *create(mPtr + aIndex);