Hey Chad I was playing with your implementation but when I try this simple setup .

[jonntd]

import maya.cmds as cmds
j = cmds.joint()
sph, _ = cmds.polySphere()
print sph
cmds.skinCluster( j, sph)
cmds.select(sph)
cmds.demBones(b=20, sf=0, ef=20, eb=j)

Traceback (most recent call last):
File "", line 1, in
File "", line 2, in demBones
RuntimeError: Maya command error #

[asdMild]

i was also get this error,
Are you solver it ?

[wujackjack]

parameter eb should be a list, it would work