--- a/mfbt/Char16.h
+++ b/mfbt/Char16.h
@@ -61,17 +61,17 @@ public:
   {
     return const_cast<wchar_t*>(reinterpret_cast<const wchar_t*>(mPtr));
   }
 
   operator const void*() const
   {
     return mPtr;
   }
-  MOZ_IMPLICIT operator bool() const
+  explicit operator bool() const
   {
     return mPtr != nullptr;
   }
 
   /* Explicit cast operators to allow things like (char16_t*)str. */
   explicit operator char16_t*() const
   {
     return const_cast<char16_t*>(mPtr);