Remove IPAM Custom Property Value/Set To Null

I have created some custom properties for IPAM objects.  Once the IPAM object is no longer used, I want to remove the values that may have been populated in these values.  For example, I created a custom property called IPAM_City.  If the location closes, I want to clear the current value in IPAM_City.  I can't seem to find a way to do this.  I'm using Python and have tried the following:

swis.update(uriIPAM_City = '')
swis.update(uri, IPAM_City = None)
swis.update(uri, IPAM_City = str())
They all give the following error,
"400 Client Error: The given key was not present in the dictionary. for url: ">"
If I use swis.update(uri, IPAM_City = 'Any Town'), it works fine so I know the key is in the dictionary and everything else it setup correctly.  For some reason it does not like it when there is not some sort of string present.
I don't want to completely remove IPAM_City as a custom property, I just want to clear/set to null the value for this specific node/subnet.