Conversion from string "" to type date is not valid

Hi

From my ssas data source I have a datetime field which is in this format 2015-01-02 00:00:00. There are also nulls in this field.

I want to change the format to dd/MM/yyy

I am using this

FORMAT(CDATE(Fields!Oldest_Action_Date.Value),"dd/MM/yyy")

When I run the report I get the warning that it won't do the conversion and any field where it is NULL is now shown with a #Error.

How can I get around this?

Thanks