Failed to find prefix for extension

module a {
  yang-version 1.1;
  namespace "a";
  prefix a;
 include b;}

submodule b {
  belongs-to a {
    prefix a;
  } import c {
   prefix c;
 }
 container foo {
     leaf bar {
       c:default-deny-all;
       type empty;
     }
  }
}

module c { 
 namespace "c";  
 prefix c;    
 extension default-deny-all {
   description "dsc";
    }
}

[ERROR] yang-to-sources: Unable to generate sources with org.opendaylight.mdsal.binding.maven.api.gen.plugin.CodeGeneratorImpl generator
java.lang.IllegalArgumentException: Failed to find prefix for statement (c)default-deny-all
    at com.google.common.base.Preconditions.checkArgument (Preconditions.java:217)
    at org.opendaylight.yangtools.yang.model.export.ExportUtils.statementPrefix (ExportUtils.java:62)
    at org.opendaylight.yangtools.yang.model.export.YangTextSnippetIterator.pushStatement (YangTextSnippetIterator.java:199)
    at org.opendaylight.yangtools.yang.model.export.YangTextSnippetIterator.computeNext (YangTextSnippetIterator.java:160)
    at org.opendaylight.yangtools.yang.model.export.YangTextSnippetIterator.computeNext (YangTextSnippetIterator.java:42)
    at com.google.common.collect.AbstractIterator.tryToComputeNext (AbstractIterator.java:141)
    at com.google.common.collect.AbstractIterator.hasNext (AbstractIterator.java:136)
    at org.opendaylight.mdsal.binding.java.api.generator.BaseTemplate.appendYangSnippet (BaseTemplate.java:425)
    at org.opendaylight.mdsal.binding.java.api.generator.BaseTemplate.appendSnippet (BaseTemplate.java:392)
    at org.opendaylight.mdsal.binding.java.api.generator.BaseTemplate.formatDataForJavaDoc (BaseTemplate.java:343)
    at org.opendaylight.mdsal.binding.java.api.generator.InterfaceTemplate.body (InterfaceTemplate.java:75)
    at org.opendaylight.mdsal.binding.java.api.generator.BaseTemplate.generate (BaseTemplate.java:88)
    at org.opendaylight.mdsal.binding.java.api.generator.InterfaceGenerator.generate (InterfaceGenerator.java:34)
    at org.opendaylight.mdsal.binding.java.api.generator.GeneratorJavaFile$GeneratorStringSupplier.get (GeneratorJavaFile.java:52)
    at org.opendaylight.mdsal.binding.java.api.generator.GeneratorJavaFile$GeneratorStringSupplier.get (GeneratorJavaFile.java:41)
    at org.opendaylight.mdsal.binding.maven.api.gen.plugin.CodeGeneratorImpl.generateSources (CodeGeneratorImpl.java:113)
    at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.generateSourcesWithOneGenerator (YangToSourcesProcessor.java:383)
    at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.generateSources (YangToSourcesProcessor.java:332)
    at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesProcessor.conditionalExecute (YangToSourcesProcessor.java:159)
    at org.opendaylight.yangtools.yang2sources.plugin.YangToSourcesMojo.execute (YangToSourcesMojo.java:127)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)