Flowfield: Lookup CalcFields Problem - Integer

infonote

Hi,

I have this particular scenario with a flowfield that uses Lookup. The field 'M ID' is an integer
Table: Sales Header - M Cust Name
Table 2: Contact.

Lookup is in the Sales Header table - 'M Cust Name'.
Lookup(Contact.name where(M ID =field(M ID))

There is a form (Sales Order) which uses this field.
Now, when the value (M ID) is empty, the field in the form displays a name even though the 'M ID' is 0.

How can I make the Name not display anything, when the M ID is empty?

I tried using 'Value is Filter' but it does not work with the integer.

Thanks in advance