debugging code with ON ERROR

[tsh73]

I just tried to step through a code with ON ERROR.
And debugger just halts with error that supposed to be caught with ON ERROR
(and it does catch it - in normal mode)

So. 
1) is there a way to debug -  step through - a code with ON ERROR "on"?
2) if not, consider that to be a wish

Sample code - second print halts in debug mode.
dim a(3)

print "a(3) is " ;  iif$(isOk("a", 3), "OK", "Not OK")
print "a(4) is " ;  iif$(isOk("a", 4), "OK", "Not OK")

function isOk(arrName$, i)
count= count+1
   on error goto [err]
   dummy=eval(arrName$;"(";i;")")
   isOk=1
   exit function
[err]
   isOk=0
end function

function iif$(test, valYes$, valNo$)
   iif$ =  valNo$
   if test then iif$ =  valYes$
end function