suprise with -=

[Ray Andrews <rayandrews@xxxxxxxxxxx>]

   test1 ()
   {
   integer first=1
   integer second=2

   ((first+=second))
   echo $first

   ((first-=second))
   echo $first

   first+=second
   echo $first

   first-=second
   echo $first

   }

   3
   1
   3
   test1:15: command not found: first-=second
   3


That's a bit of a surprise, why is zsh fussier with '-=' than with '+='?