Access a field of Main Form from a SubForm 1

[matrec]

Hi, I have a main Form called "frmMain" and a subform called "subFrm", there common field is NotNo.
How can I check that a text field called "eacNo", which belongs to "frmMain" is Null, from the on-entry event of field "fName" of the subform "subFrm" ?
I have tried

if Len(Forms!frmMain!eacNo)=0
but the error messages says that frmMain cannot be found.
Any clues to solve the problem ?
Thank you

 

[BSman]

The easiest way is to use the "parent". So to check the status of the control eacNo on the main form from the subform you would type (within the subform):

isnull(me.parent.eacno)

 

[matrec]

Thanks BSman.
Solved the problem
Rgds