JMXGenerator IllegalStateException when referring a Grouping from augmented model

I have 2 yang files when constructing a system entity:

The main file a "system.yang" where the entity-fields are defined, and configuration file "system-config.yang" that provides an augmentation to configuration with binding-broker, data-provider and rpc-registry containers inside (standard definition):

augment "/config:modules/config:module/config:configuration" {

An additional container is appended after "container rpc-registry" e.g.

container entity-config {
uses system:entity-fields;

leaf description

{ type string; }

}

If I removed the "leaf description" from the "system-config.yang", I'm getting a following ERROR. If the "leaf description" remains no ERROR is thrown and entity's fields are generated:

[ERROR] Failed to execute goal org.opendaylight.yangtools:yang-maven-plugin:0.6.2-SNAPSHOT:generate-sources (default) on project ..: yang-to-sources: One or more code generators failed, including failed list(generatorClass=exception)

{org.opendaylight.controller.config.yangjmxgenerator.plugin.JMXGenerator=java.lang.IllegalStateException}

-> [Help 1]

Question is why there should be an additional leaf defined?