DMax on mainform referencing subform

Hi I have a form - purchase, and a subform purchasedetailds

they are linked with a field "puid"

I need the dmax to give me the maximum auto number field "pdid" on the subform in a textbox on the mainform.

I used the control source code:
=DMax("[forms]![purchase]![purchasedetailds]![pdID]","[forms]![purchase]![purchasedetailds]","[Forms]![purchse]![purchasedetailds]![puid]=" & [Forms]![purchase]![puid])

and the simpler:
=DMax("purchasedetailds]![pdID]","[purchasedetailds]","[purchasedetailds]![puid]=" &[puid])

and i get the #error in the box can any one see what i am doing wrong?
thanks
Simon

DMax on mainform referencing subform