Customs Expressions in Lookup Wizard for Edit Page

Hi,
I'm trying to build a lookup wizard with this custom expression in the display field:
CONCAT (FIELD1,', ',FIELD2,'. ',FIELD3)
but I receive the following message "Failed to obtain data from database. Check out for errors in the custom expression".
If I insert the same expression but with two fields only, then all it's ok. So, can you help me?...
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