Trying to assign a policy to a virtual server

i am trying to change policies on a virtual server, using the python sdk.

i load the vs, i can load the existing policy on the vs, and delete it, but when i try to create a new policy for the vs, i get a 404 error saying that the policy cannot be found.

##  load the virtual server
vs = mgmt.tm.ltm.virtuals.virtual.load(name="serviceMain", partition ="partition", subPath = "subPath")

## load the policy.  this works
policy = mgmt.tm.ltm.policys.policy.load(name="pol_name", partition ="partition", subPath = "subPath")

##  load existing policy from the vs and delete it.  this works
pol = vs.policies_s.policies.load(name="pol_name", partition ="partition", subPath = "subPath")
pol.delete()

##  create a new policy.  this fails
mgmt.tm.ltm.policys.policy.create(name="pol_name", partition ="partition", subPath = "subPath")

i get this error:

Text: '{"code":404,"message":"01020036:3: The requested policy (pol_name) was not found.","errorStack":[],"apiError":3}'

the policy exists, and was loaded earlier in the script, but it can't be found.  i have tried other methods of attaching the policy, and they have all failed. 

any ideas?

thanks,

-t