Cell to show blank until data entered ?

Hi,

I have the following formula.

=IF(O48="N",'Constant-Anaylsis'!$C$7,IF(O48="C",'Constant-Anaylsis'!$C$9,IF(O48="P",'Constant-Anaylsis'!$C$11,IF(O48="EP",'Constant-Anaylsis'!$C$13,0))))

Problem is, the cell which contains the formula(s) automaticaly displays 9.50 (which is C7 under Constant-Analysis etc.). Obviously, this changes if you enter either N, C, P, EP etc. but I don't want the cell to display anything until you enter the relevant code (N, C, P, EP etc.).

?

Thanks.