I believe @MartinSpence’s question is because in the image you uploaded, it appears that you are inputting a family type called “Wall Panel Interior Top Ledge” which could either be some unusual Titleblock naming or it is not a title block family (which would explain your initial error)
This is normal behavior for Dynamo and can be avoided if you create your script to run via the Dynamo Player but there are other resolutions that can be found on the forum - I believe it is the same behavior described here