Syntax for a filter for an import from CX Sales into Eloqua

I am wondering with the syntax is for filtering on a custom field in CX Sales within Eloqua.

For a standard field, I use this

LastName='Ellison'

and it works

But when I try a custom field

SalesRole_c='FieldSales' and

and I get this result

Error: {"errorCode":"400", "errorMessage":"BAD_REQUEST"}

What am I missing