AWS Quicksight if else with dates

I am trying to merge two date columns together with the ifelse function, my formula is

ifelse(RewardPaidDate = null,rewarddate, RewardPaidDate)

however it only returns the RewardPaidDate column and leaves the nulls in the new calculated field named RewardSentDate. I have tried changing it to

ifelse(RewardPaidDate = '',rewarddate, RewardPaidDate)

but I get the error message: "At least one of the arguments in this function does not have correct type. Correct the expression and choose Create again."

calculated column is RewardSentDate