Workflow Error: Coercion Failed: Input cannot be null for this coercion

I have a workflow that is assigning a task based on a lookup value. This workflow runs successfully the first time then fails on subsequent attempts. Here's how the data looks:

Look Up

 Title     Contacts
-------   ----------
 Plant1    John Smith; Jane Smith
 Plant2    John Doe; Jane Doe

Workflow List

 Title    Desc    Plant
-------  ------  -------
Test 1    asdf    Plant1

In the workflow, the plant contacts are found like this:

 Retrieve Data
---------------
 Plants -> Contacts as Email Semicolon Delimited

 Find Data
---------------
 Plants:Title = CurrentItem:Plant

Why would it work once and then fail? I have isolated this as the issue because if I hard code a Plant Title it works every time.