Value of expression '0' cannot be used as the target of an assignment

In one of my custom workflow steps, there is an error "Value of expression '0' cannot be used as the target of an assignment". It implies that a binding error occurred in runtime the value couldn't be assigned to the target.

However, I removed all bindings from the binding editor and still the same error. What could cause this?

I am in ECC 6.0.

Appreciate it...