ODL fails to mount device - shows empty list of unsatisfied imports

when mounting some types of device I'm seeing the mount fail with "No more sources for schema context'.

I see this in the logs:

2016-11-29 17:39:00,580 | DEBUG | ssing-executor-9 | NetconfDevice | 315 - org.opendaylight.netconf.sal-netconf-connector - 1.4.1.Boron-SR1 | RemoteDevice

{3850}: Unable to map any source identifiers to a capability reported by device : []
2016-11-29 17:39:00,580 | WARN | ssing-executor-9 | NetconfDevice | 315 - org.opendaylight.netconf.sal-netconf-connector - 1.4.1.Boron-SR1 | RemoteDevice{3850}

: Unable to build schema context, unsatisfied imports {}, will reattempt with resolved only
2016-11-29 17:39:00,581 | DEBUG | ssing-executor-9 | NetconfDevice | 315 - org.opendaylight.netconf.sal-netconf-connector - 1.4.1.Boron-SR1 | {}: Unable to build schema context, unsatisfied imports {}, will reattempt with resolved only
SchemaResolutionException{unsatisfiedImports={}}
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)[:1.8.0_111]
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)[:1.8.0_111]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)[:1.8.0_111]
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)[:1.8.0_111]
at org.opendaylight.yangtools.util.concurrent.ReflectiveExceptionMapper.newWithCause(ReflectiveExceptionMapper.java:31)[47:org.opendaylight.yangtools.util:1.0.1.Boron-SR1]
at org.opendaylight.yangtools.util.concurrent.ExceptionMapper.apply(ExceptionMapper.java:87)[47:org.opendaylight.yangtools.util:1.0.1.Boron-SR1]
at org.opendaylight.yangtools.util.concurrent.ExceptionMapper.apply(ExceptionMapper.java:37)[47:org.opendaylight.yangtools.util:1.0.1.Boron-SR1]
at com.google.common.util.concurrent.Futures$MappingCheckedFuture.mapException(Futures.java:1809)[44:com.google.guava:18.0.0]
at com.google.common.util.concurrent.AbstractCheckedFuture.checkedGet(AbstractCheckedFuture.java:85)[44:com.google.guava:18.0.0]
at org.opendaylight.netconf.sal.connect.netconf.NetconfDevice$SchemaSetup.setUpSchema(NetconfDevice.java:452)[315:org.opendaylight.netconf.sal-netconf-connector:1.4.1.Boron-SR1]
at org.opendaylight.netconf.sal.connect.netconf.NetconfDevice$SchemaSetup.run(NetconfDevice.java:428)[315:org.opendaylight.netconf.sal-netconf-connector:1.4.1.Boron-SR1]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)[:1.8.0_111]
at java.util.concurrent.FutureTask.run(FutureTask.java:266)[:1.8.0_111]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)[:1.8.0_111]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)[:1.8.0_111]
at java.lang.Thread.run(Thread.java:745)[:1.8.0_111]
Caused by: java.lang.NullPointerException

So it seems that ODL thinks it has unsatisfied imports, but that the list of such imports is empty.

any idea what's going on here?