Error validating formula for calculated column

Question

I'm trying to make up a calculated column that evaluates an existing column with a CASE statement. The following code works fine:

CASE 
  WHEN Technician_Production_date IS NULL THEN 1 
  WHEN Technician_Production_date IS NOT NULL AND Technician_Review_Date IS NULL THEN 2 
  WHEN Technician_Production_date IS NOT NULL AND Technician_review_Date IS NOT NULL AND 
  Chemist_Review_Date IS NULL THEN 3 
  ELSE 4 
END

When i try to do the same statement but assign an existing column as a VARCHAR value instead as below, i get the error: "Error validating formula for (column_name)". Here is the code im trying to run:

CASE 
  WHEN Technician_Production_date IS NULL THEN CAST(Responsible_Technician_Review AS varchar) 
  WHEN (Technician_Production_date IS NOT NULL AND Technician_Review_Date IS NULL) THEN 
  CAST(Responsible_Chemist_Review AS varchar) 
  WHEN (Technician_Production_date IS NOT NULL AND Technician_review_Date IS NOT NULL AND 
  Chemist_Review_Date IS NULL)  THEN CAST(Responsible_Chemist_Approval AS varchar) 
  ELSE "Finished") 
END

What could be causing this?