Sum Network days but if field is blank results show invalid data type

The subject pretty well tells it all,

I am trying to sum the network days between the request received and request sent date but if we haven't sent the request response as yet the field is empty and shows the error " Invalid data type"

Current sum is

=NETWORKDAYS([Quote Request Recieved]198, [Quote Sent Date]198)


Can someone please assist in including if the sent field isnt complete yet to show "0"? Thank you 


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