sourcing a sh file in zsh

[Evert Rol <evert.rol@xxxxxxxxx>]

  Hi,

How do I source a .sh file in zsh? It works fine in bash or sh, but  
gives an error when source in zsh.

(The error itself, for example is as follows:

$> source bla.sh
bla.sh:1: bad math expression: operand expected at `'


where that line reads:
  [ "$a_root" = "" ] && a_root=/some/dir

Note that it's a lengthly script, and I didn't write it, so I'm not  
keen on changing everything to make it zsh compatible.
)

Thanks,

  Evert