sql - trying to update a column with difference between 2 dates when one can be NULL -

with following code:

update factquote   set conversiondays =    case     when        conversiondate = null null     else       datediff(day, initialcontactdate, conversiondate)   end 

keep getting message:

msg 206, level 16, state 2, line 10 operand type clash: int incompatible date 

how can fix this?

this not answer, simplification (which long comment).

datediff() -- many other functions -- returns null when either date argument null. so, can simplify expression to:

update factquote   set conversiondays = datediff(day, initialcontactdate, conversiondate);