issue to find month name

In SSRS report need to display monthname. The month field may have null value, if it has null value value return value should be empty.

In database prog_mnth value maybe 1,4,7,null,2. Should be display in report like Jan,Apr,Jul, ,Feb

I tried this,

iif(len(Fields!prog_mnth.Value)>0,MonthName(Fields!prog_mnth.Value),"")

But it gives #error in the column where prog_mnth field is null. If i try iif(len(Fields!prog_mnth.Value)>0,Fields!prog_mnth.Value,"Not Been set"), its returning correct value. When i try to use MonthName() only its giving error.

Please give idea how to return monthname value if the field doesnt have value need to return empty value.

Thank you.