Compile Error: Invalid field using Rich Text with a Test Class

We recently added a validation rule that requries a Rich Text Field to be filled in when the object is completed. This object is "Implementation" and the field is "Transition_Notes__c"

In my test classes where I have completed Implementation objects I've attempted to add Transition_Notes__c so the Validation rule doesn't impact my test class.

When I do this, I get "

Error: Compile Error: Invalid field Transition_Notes__c for SObject Implementation__c at line 56 column 9

Normally I see this error when I make a typo. I've tried copy and pasting the API name. I've done this using the Apex Class GUI editor in Salesforce. I also tried the IDE in Eclipse using the auto-complete feature. And I still get this error.

Here's a snippet:

Implementation__c CompletedI1 = new Implementation__c();

        CompletedI1.Stage__c = 'Live (completed)'; 
        CompletedI1.Status__c = 'Completed';
        CompletedI1.Actual_live_Date__c = date.Today();
        CompletedI1.Completed_Date__c = date.Today();
        CompletedI1.Imp_Survey_Recipient__c = c1.id;
        CompletedI1.Transition_Notes__c = 'Filled in';
        update CompletedI1;

I know the API name is right. I know it's attached to the right variable of the SObject. I can see the field in the Schema. What's going on?