Why can't I set my AI Character BP's AI Controller Class?

I am having trouble with my AI Character. I have made an AI Controller class called “AI_Control”. I have made a AI Character Blueprint class which is a duplicate of ThirdPersonCharacter Blueprint. When I go into the AI_Character Blueprint to Class Defaults > Pawn > AI Controller Class I click the drop down to set it to be “AI_Control”, but instead after clicking it it reads “None” as the setting instead. I can’t seem to fix this. Please help.