error at left operand?

[Esa Kivelä]

I have code:

on idle
  set the itemdel to ","
  put item 1 of field "huu" + item 1 of field "buu" + item 1 of field "suu" into field "28"

end idle

and I get error message: "  * There was an Execution Error at 10:58:55 AM
Error description:  Operators +: error in left operand
Object: field id 1066 of card id 1002 of stack "C:/Program Files/Revolution 1.1.1/resusrssikortti.rev"
--------------------
put item 1 of field "huu" + item 1 of field "buu" + item 1 of field "suu" into field "28"
--------------------"

What that mean?

EsaK