diff --git a/mfbt/JSONWriter.h b/mfbt/JSONWriter.h
index 98747b15ba0..1aa83043c8c 100644
--- a/mfbt/JSONWriter.h
+++ b/mfbt/JSONWriter.h
@@ -145,7 +145,7 @@ class JSONWriter
     }
 
   public:
-    EscapedString(const char* aStr)
+    explicit EscapedString(const char* aStr)
       : mUnownedStr(nullptr)
       , mOwnedStr(nullptr)
     {