UnityEngine.DefautAsset as TextAsset

I'm trying to load a CSV file using a relative reference as a TextAsset.

 dataBase = AssetDatabase.LoadMainAssetAtPath("Assets/Game Specific Assets/GameData/weapons.csv") as TextAsset;

But I get an invalid cast exception. I can find no reference to UnityEngine.DefaultAsset anywhere and I'm not sure what I'm doing wrong.