If error in UDF

I have data in col 1
a1= alex berbec
a2 = asoi
a3= john travolta
Etc

 Function UD(r as range)
UD = split(r.value, " ") (1)
End function

I want take split first split
In b1= berbex
In b2 =asoi
In b3 = travolta
But in A2 = alex udf will be error
How to else my udf

function UD ( r as range)
UD =iif(iserror(UD),r.value,split(r.value, " ")(1))
End function

But still error