[Jprogramming] invoke rank 0 verb with 0-length vector

[bill lam]

I test why 1+'' does not cause domain error and eventually come up with
   foo=: (4 : 0)"0
1!:2&2 'foo'
1 + ' '
)
   1 + ' '
|domain error
|   1    +' '


   1 foo ''
foo

The body of foo is executed once but it does not cause domain error.  How to 
explain this behaviour?
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