diff --git a/gfx/thebes/gfxPrefs.h b/gfx/thebes/gfxPrefs.h
index 0ad821e64522..decb6de19d4d 100644
--- a/gfx/thebes/gfxPrefs.h
+++ b/gfx/thebes/gfxPrefs.h
@@ -137,7 +137,7 @@ private:
   class TypedPref : public Pref
   {
   public:
-    TypedPref(T aValue)
+    explicit TypedPref(T aValue)
       : mValue(aValue)
     {}