[AccessD] #Func! in Access 2010

Does anyone know what #Func! means? I can't find a reference to it anywhere. 

In Access 2010, I'm using a simple parsing expression: 

FirstName: Left([Name],InStr([name]," ")-1)

and this value shows up just below the field header in Datasheet view. If I remove the -1 component, #Func! disappears. 

Anybody know what this is? 

Susan H.