accessing a value in a multidimensional associative array

I'm having some troubles with this one.  I set up my array like so:

$POs[] = array(
  $jobID=>array(
  'FSME' => $matTotal,
  'FSMC' => $matTotal,
  'FSDE' => $draftingTotal,
  ...

And say $jobID is equal to 312.

Now I am trying to access one of those values.  I tried:

$POs[$jobID]['FSME']

with $jobID being 312.  Then I get "Notice: Undefined offset: 312 in..."  Any ideas how I can access this variable?

Thanks

Mike