My Display Method gives me an error message

Hi all,

Here below is the code for my Display method, it gives me an error message while it shouldn't. I looked at the code and it should be fine, it tells me: "The operand is not compatible with the type of the function". The 2 tables are already present in the report so I didn't need to declare any of them. Whe I try to use the table and field in the property without doing the Display method the field appeared but it was empty. So I decided to use a display method that would activate each time AssetGroup.RecId is equal to AssetTable.RecId.

Display AssetTable Location()
{
   Select Location FROM AssetTable
   Where AssetGroup.RecId == AssetTable.RecId;
   Return AssetTable.Location;

}

I am sure I have made a mystake but I don't know why. If someone cal help me to fix that code it would be great.

Regards