NPE running the enforcer plugin

I get a NPE when using the enforcer plugin:

[INFO] [enforcer:enforce

]
[INFO] Checking unresolved references to org.codehaus.mojo.signature:java16:1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[ERROR] FATAL ERROR
[INFO] ------------------------------------------------------------------------
[INFO] null
[INFO] ------------------------------------------------------------------------
[INFO] Trace
java.lang.NullPointerException
at org.codehaus.mojo.animal_sniffer.enforcer.CheckSignatureRule.execute(CheckSignatureRule.java:107)
at org.apache.maven.plugins.enforcer.EnforceMojo.execute(EnforceMojo.java:185)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:490)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:694)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:556)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:535)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.java:387)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:348)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:180)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:328)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:138)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:362)
at org.apache.maven.cli.compat.CompatibleMain.main(CompatibleMain.java:60)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:613)
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)

my configuration is this:
<build>
<plugins>
<plugin>
<artifactId>maven-enforcer-plugin</artifactId>
<version>1.0-beta-1</version>
<dependencies>
<dependency>
<groupId>org.codehaus.mojo</groupId>
<artifactId>animal-sniffer-enforcer-rule</artifactId>
<version>1.3</version>
</dependency>
</dependencies>
<executions>
<execution>
<id>enforce-versions</id>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<requireJavaVersion>
<version>[1.7.0,)</version>
</requireJavaVersion>
</rules>
</configuration>
</execution>

<execution>
<id>check-java16</id>
<phase>test</phase>
<goals>
<goal>enforce</goal>
</goals>
<configuration>
<rules>
<checkSignatureRule implementation="org.codehaus.mojo.animal_sniffer.enforcer.CheckSignatureRule">
<signature>
<groupId>org.codehaus.mojo.signature</groupId>
<artifactId>java16</artifactId>
<version>1.0-SNAPSHOT</version>
</signature>
</checkSignatureRule>
</rules>
</configuration>
</execution>
</executions>
</plugin>