Left field type is string, right field type BIGINT

Left field type is string, right field type BIGINT

Dear,

I am trying to look up a value in a form field (field A), based on input in another form field (field B).
In the "On user input" field of field B, I have the following code :

varA  =  Status_action  [Status_name == input.Status_name];
input.Status_ID = varA.ID;

However, I get an error that the field type of the field on the left is String, where the field type on the right is BIGINT. 
(in reality, the field on the left is a lookup field, looking up the field on the right; hence I would think they have the same field type).

what should I do ?