NPE if <requireFilesExist> contains an unset variable

With this snippet of metadata:
<requireProperty>
<property>ndbj.jnilib2</property>
<message>"You must have a ndbj.jnilib2!"</message>
</requireProperty>
<requireFilesExist>
<files>
<file>${ndbj.jnilib2}</file>
</files>
</requireFilesExist>

the variable ndbj.jnilib2 doesn't exist and results in:

java.lang.NullPointerException
at org.apache.maven.plugins.enforcer.RequireFilesExist.checkFile(RequireFilesExist.java:38)
at org.apache.maven.plugins.enforcer.AbstractRequireFiles.execute(AbstractRequireFiles.java:71)
at org.apache.maven.plugins.enforcer.EnforceMojo.execute(EnforceMojo.java:185)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:443)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:539)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:480)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:459)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:311)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:278)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:143)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:334)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:125)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:272)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)