Operand error...

$oppdater_lager_med_antall_kg = $kg_pa_lager +
$kg_fra_transportdokument_inn_pa_valgt_lager;


result:
*Fatal error*: Unsupported operand types in *
/Users/karl/Sites/kasen/io/kp/index.php* on line *2970

*I have also tried this:
$kg_pa_lager += $kg_fra_transportdokument_inn_pa_valgt_lager;

Both of them return this Fatal error.
I am using Dreamweaver CS6 and the syntax check in my software say: "No
syntax error"

What am I doing wrong this time?

Thanks for your good advice!

Karl