Insight to 'Null argument not allowed' error

user(pv!servicesDelivered.iss_analyst,"firstName")& " " & user(pv!servicesDelivered.iss_analyst,"lastName")

Above is the code I have, simple user fuction to grab first and last name for user. 

Currently getting this error: 

(Expression evaluation error at function 'user': Null argument not allowed) (Data Inputs)

Any insight to why? 

Than you in advance! 

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