roslib.message.get_message_class returns None

edit

I'm trying to create a tool that can dynamicly read service call descriptions from a yaml file and call the services accordingly. (in rospy on kinetic)

If i try to resolve a non-standard service type form a string, roslib.message.get_message_class returns None. Why does this happen? Shouldn't it either throw an error, like when the string doesn't mean anything, or return the correct type?

try:
  msg_type = roslib.message.get_message_class('somethingstupid')
  if msg_type is None:
    rospy.logwarn('msg_type is None')
except:
  rospy.logerr('roslib.message.get_message_class failed')

results in

roslib.message.get_message_class failed

while

try:
  msg_type = roslib.message.get_message_class('my_package/MyMsgType')
  if msg_type is None:
    rospy.logwarn('msg_type is None')
except:
  rospy.logerr('roslib.message.get_message_class failed')

results in

msg_type is None