RESTCONF rpc() input XML fails while json equivalent succeeds

POST to http://localhost:8181/restconf/operations/network-topology:network-topology/topology/topology-netconf/node/47-netconf-device/yang-ext:mount/ietf-netconf:get-config

<input xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf">
<source>running</source>
</input>
...
<error-type>protocol</error-type>
<error-tag>malformed-message</error-tag>
<error-message>Error parsing input: Unknown child(ren) node(s) detected, identified by: (urn:ietf:params:xml:ns:yang:ietf-netconf)source, in: container input</error-message>

Try POST :
<input xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf">
<source><running/></source>
</input>
... same error
<error-type>protocol</error-type>
<error-tag>malformed-message</error-tag>
<error-message>Error parsing input: Unknown child(ren) node(s) detected, identified by: (urn:ietf:params:xml:ns:yang:ietf-netconf)source, in: container input</error-message>

Switch to json:

{ "input": { "source" :

} }

Works!