Coercion Failed: Unable to transform the input lookup data into the requested type.

Hello,

I have a CUSTOM LIST and I add a new item by code (Visual Studio). I can see New Item's all fields. I start an APPROVAL WORKFLOW with that item (list workflow) but It doesn't start. It says CANCELLED and this error:

Coercion Failed: Unable to transform the input lookup data into the requested type.

How can I solve this problem?

Thanks.