Form Modal problem - programme continues

[scoop10000]

Why is it that the following code works perfectly well. Within my program the form opens and waits for my selection.

DoCmd.OpenForm "mainCustomerForm",, , "[PostCode]='" & customer & "'", , acDialog


However when I change it to datasheet view the program continues on through failing to stop

DoCmd.OpenForm "mainCustomerForm", acFormDS,, "[PostCode]='" & customer & "'", , acDialog

Am I missing something here