array_key_exists() expects parameter 2 to be array, boolean given

Line 318
function workflow_transition($node, $sid, $force = FALSE) {
// Make sure new state is a valid choice.
if (array_key_exists($sid, workflow_field_choices($node, $force))) {
If there are no choices for a user then this line will throw a wobbly.
The fix would be to check choices first with something like:

if (workflow_field_choices($node, $force) == FALSE) {
  return;
}