Maven-plugin build fails with missing version of org.codehaus.plexus:plexus-classworlds

[INFO] --- maven-enforcer-plugin:1.0.1:enforce (enforce) @ shrinkwrap-resolver-maven-plugin ---
[WARNING] Rule 0: org.apache.maven.plugins.enforcer.DependencyConvergence failed with message:
For artifact {org.codehaus.plexus:plexus-classworlds:null:jar}: The version cannot be empty.
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] ShrinkWrap Resolver Bill of Materials ............. SUCCESS [4.832s]
[INFO] ShrinkWrap Resolver Aggregator .................... SUCCESS [27.086s]
[INFO] Shrinkwrap Resolver Build Resources ............... SUCCESS [8.974s]
[INFO] ShrinkWrap Resolver API ........................... SUCCESS [20.971s]
[INFO] ShrinkWrap Resolver SPI ........................... SUCCESS [4.081s]
[INFO] ShrinkWrap Resolver Maven API ..................... SUCCESS [12.928s]
[INFO] ShrinkWrap Resolver Maven SPI ..................... SUCCESS [1.175s]
[INFO] ShrinkWrap Resolver Maven Archive API ............. SUCCESS [1.969s]
[INFO] ShrinkWrap Resolver Maven Implementation .......... SUCCESS [4:52.803s]
[INFO] ShrinkWrap Resolver Maven Archive SPI ............. SUCCESS [1.070s]
[INFO] ShrinkWrap Resolver Maven Archive Implementation .. SUCCESS [1:22.254s]
[INFO] ShrinkWrap Resolver Dependency Chain .............. SUCCESS [2.726s]
[INFO] ShrinkWrap Resolver Maven Plugin .................. FAILURE [1.540s]
[INFO] ShrinkWrap Resolver Embedded Gradle Archive API ... SKIPPED
[INFO] ShrinkWrap Resolver Embedded Gradle Archive Implementation  SKIPPED
[INFO] ShrinkWrap Resolver Gradle Dependency Chain ....... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 7:48.587s
[INFO] Finished at: Mon Aug 22 07:35:14 EDT 2016
[INFO] Final Memory: 28M/236M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.0.1:enforce (enforce) on project shrinkwrap-resolver-maven-plugin: Some Enforcer rules have failed. Look above for specific messages explaining why the rule failed. -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR] 
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <goals> -rf :shrinkwrap-resolver-maven-plugin
Build step 'Invoke top-level Maven targets' marked build as failure
Recording test results
Finished: FAILURE