I was encountering this error in a FIM Workflow with an If-Else statement that was using a Declarative Rule Condition where the declarative rule condition name was the “condition name”.
The fix was to add something like the following in the code behind:
private void ccManagerEmailFound(object sender, ConditionalEventArgs e)
e.Result = bManagerEmailFound;
Then, change the If-Else to a Code Condition and select the new code condition just added.