The javadoc [1] for javax.ws.rs.core Configurable interface says that:

"An existing property can be removed by assigning a null value to the property. "

ClientWebTarget.property(some_property, null) throws a null pointer exception instead of clearing the property value.

ResteasyProviderFactory.property() method can remove the property if the value of the property is null (this was fixed in 3.0.12-Final via https://issues.jboss.org/browse/RESTEASY-1184), but ClientWebTarget remains unchanged and does not take advantage of this ability.

[1] http://docs.jboss.org/resteasy/docs/3.0.9.Final/javadocs/index.html