Problems with if statement in User Formula field

I am trying to use an if statement in a user formula field and it doesn't seem to be working. It is designed to determine whether a checkbox is checked, then outputs a user field value based on the condition.

If ([my checkbox]=true,
  [user field A],
  [user field B (via relationship)]
)

I am getting the following error with the field in the else output highlighted:

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Matt Makris
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