Issue with Calculated column

[Kolumam]

Hi All,

I wrote a formula to show NA if it is blank and if the date is below 31/3/2018, but I am getting the error

Expressions that yield variant data-type cannot be used to define calculated columns.

Specific Water Usage (litres/kWp) = 
IF('Cleaning Activity'[Specific_Water_Usage__c]<>BLANK(),
'Cleaning Activity'[Specific_Water_Usage__c],
IF('Cleaning Activity'[Date_of_Cleaning__c]<DATE(2018,3,31),
"NA",'Cleaning Activity'[Specific_Water_Usage__c]))

Can you please tell me how to put it in another way?