Report Viewer IiF statemant

i want to use IIf statement this way :

i have A1 column 

iif(Fields!a1.Value =nothing , "00:00" , Fields!a1.Value.substring(0,1))

=IIF(isnothing(Fields!a1.Value), "00:00", Fields!a1.Value.substring(0,1))

when a1 is empty, i am getting error. other way i am getting  this value "Fields!a1.Value.substring(0,1)".  

how to resolve this error? 

i have tried "Fields!a1.Value < "0"" or  "Fields!a1.Value > nothing"  "Fields!a1.Value.tostring  and many other things but when value is null (A1 is empty )  i am getting error in reportviewer A1 column 

thanks