Make Task Sub Type required when Task Type = Send Literature

Hell Trailblazers,

I am trying to create a validation rule to make the task Sub-Type field mandatory when Task Type = Send Literature. Below is my formula:

AND( ISPICKVAL(Type , 'Send Literature'), ISBLANK(TaskSubtype ) )

However I am getting the following error:  Error: Field TaskSubtype is a picklist field. Picklist fields are only supported in certain functions

How can I fix this, please guide.

Thanks,

Fahad

#Trailhead