diff --git a/mfbt/Char16.h b/mfbt/Char16.h
index 380303a925f0..70dbf8a2c1eb 100644
--- a/mfbt/Char16.h
+++ b/mfbt/Char16.h
@@ -66,7 +66,7 @@ public:
   {
     return mPtr;
   }
-  MOZ_IMPLICIT operator bool() const
+  explicit operator bool() const
   {
     return mPtr != nullptr;
   }