Index creation fails when classpath is set in manifest

The following plugin configuration fails deterministically, as JarArchiver tries to open a directory folder as a RandomAccessFile, which fails (at least on Windows):

<plugin>
<artifactId>maven-jar-plugin</artifactId>
<configuration>
<archive>
<index>true</index>
<manifest>
<addClasspath>true</addClasspath>
</manifest>
</archive>
</configuration>
</plugin>

The error is:
$ mvn clean package -e
[...]
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Error assembling JAR

Embedded error: Problem creating jar: d:\adesso\eclipse-Arbeitsbereiche\Portale2-Test1\indextest\target\classes (Zugriff
verweigert)
[INFO] ------------------------------------------------------------------------
[INFO] Trace
org.apache.maven.lifecycle.LifecycleExecutionException: Error assembling JAR
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:583)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLifecycle(DefaultLifecycleExecutor.java:49
9)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(DefaultLifecycleExecutor.java:478)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHandleFailures(DefaultLifecycleExecutor.jav
a:330)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegments(DefaultLifecycleExecutor.java:291)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLifecycleExecutor.java:142)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:336)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:129)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:287)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:79)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:618)
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)
Caused by: org.apache.maven.plugin.MojoExecutionException: Error assembling JAR
at org.apache.maven.plugin.jar.AbstractJarMojo.createArchive(AbstractJarMojo.java:225)
at org.apache.maven.plugin.jar.AbstractJarMojo.execute(AbstractJarMojo.java:237)
at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPluginManager.java:451)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(DefaultLifecycleExecutor.java:558)
... 16 more
Caused by: org.codehaus.plexus.archiver.ArchiverException: Problem creating jar: d:\adesso\eclipse-Arbeitsbereiche\Porta
le2-Test1\indextest\target\classes (Zugriff verweigert)
at org.codehaus.plexus.archiver.AbstractArchiver.createArchive(AbstractArchiver.java:690)
at org.apache.maven.archiver.MavenArchiver.createArchive(MavenArchiver.java:421)
at org.apache.maven.plugin.jar.AbstractJarMojo.createArchive(AbstractJarMojo.java:218)
... 19 more
Caused by: java.io.FileNotFoundException: d:\adesso\eclipse-Arbeitsbereiche\Portale2-Test1\indextest\target\classes (Zug
riff verweigert)
at java.io.RandomAccessFile.open(Native Method)
at java.io.RandomAccessFile.<init>(RandomAccessFile.java:243)
at org.codehaus.plexus.archiver.zip.ZipFile.<init>(ZipFile.java:147)
at org.codehaus.plexus.archiver.zip.ZipFile.<init>(ZipFile.java:132)
at org.codehaus.plexus.archiver.jar.JarArchiver.grabFilesAndDirs(JarArchiver.java:922)
at org.codehaus.plexus.archiver.jar.JarArchiver.createIndexList(JarArchiver.java:499)
at org.codehaus.plexus.archiver.jar.JarArchiver.finalizeZipOutputStream(JarArchiver.java:422)
at org.codehaus.plexus.archiver.zip.AbstractZipArchiver.execute(AbstractZipArchiver.java:245)
at org.codehaus.plexus.archiver.AbstractArchiver.createArchive(AbstractArchiver.java:673)
... 21 more
[...]

See also minimum test project attached.