** ERROR: Link failed (source linkable is invalid)

[mhead10]

What does the following error mean (I cannot find much help on the issue):

** ERROR: Link failed (source linkable is invalid)

this happens when I try to load a .dae file (which does load if it's the only code)

def openGrasp():
	global marker
	global link_1
	global devices
	global link_2
	global ob
		
	if link_2:
		link_2.remove()
		link_2 = None
		link_1.remove()
		link_1 = None
		marker.remove()
		pos = device.getPosition()
		rot = device.getQuat()
		print 'openGrasp1'
		marker = viz.add('open_babcock.dae')
		marker.translate(pos)
		marker.rotatequat(rot)
		marker.setScale(0.8,0.8,0.8)
		link_1 = viz.link(device,marker)
		
	else:
		link_1.remove()
		link_1 = None
		marker.remove()
		pos = device.getPosition()
		rot = device.getQuat()
		print 'open babcock2'
		marker = viz.add('open_babcock.dae')
		marker.translate(pos)
		marker.rotatequat(rot)
		marker.setScale(0.8,0.8,0.8)
		link_1 = viz.link(device,marker)

openGrasp()

The interactive display is as follows:

open babcock2
Loading File: open_babcock.dae
** ERROR: Link failed (source linkable is invalid)

Thank you in advance for any help.