Report Builder 3.0 #Error with Date Calcs

I am using Report Builder 3.0.  In one of my columns, I'm pulling a date from a cube.  A few entries come back with a value of "Unknown".  I'm trying to use the following formula to replace "Unknown" with a blank space.

=Iif(Fields!Test_Rep_Recd_Date.Value="Unknown","",cdate(Fields!Test_Rep_Recd_Date.Value))

I've also tried:

=Iif(IsDate(Fields!Test_Rep_Recd_Date.Value),cdate(Fields!Test_Rep_Recd_Date.Value),"")

Both formulas come back with "#Error" if the value returned from the cube is "Unknown".  If the value is a date, it converts it to a date which lets me change the format of the date.

Any ideas as to why these formulas would return "#Error" instead of the value specified in the IIF statement?