Maven plugin throws when loading project unless <con:interface> removed

I'm using Maven plugin soapui-maven-plugin version 5.6.0 (latest as of this writing), but this happens exactly the same regardless of version as far back as I can test and on soapui-pro-maven-plugin version 5.1.2 (also latest).

<plugin>
  <groupId>com.smartbear.soapui</groupId>
  <artifactId>soapui-maven-plugin</artifactId>
  <version>5.6.0</version>
  <configuration>
    <projectFile>REST-Project-1-readyapi-project.xml</projectFile>
  </configuration>
</plugin>

When run, I get this output (see attachment for whole log, this is just the snippet):

[DEBUG] Configuring mojo com.smartbear.soapui:soapui-maven-plugin:5.6.0:test from plugin realm ClassRealm[plugin>com.smartbear.soapui:soapui-maven-plugin:5.6.0, parent: jdk.internal.loader.ClassLoaders$AppClassLoader@4e0e2f2a]
[DEBUG] Configuring mojo 'com.smartbear.soapui:soapui-maven-plugin:5.6.0:test' with basic configurator -->
[DEBUG]   (f) globalProperties = []
[DEBUG]   (f) projectFile = REST-Project-1-readyapi-project.xml
[DEBUG]   (f) projectProperties = []
[DEBUG]   (f) soapuiProperties = {}
[DEBUG] -- end configuration --
SoapUI 5.6.0 Maven2 TestCase Runner
10:27:35,357 INFO  [DefaultSoapUICore] initialized soapui-settings from [C:\Users\jlopacinski\soapui-settings.xml]
10:27:35,940 INFO  [PluginManager] 0 plugins loaded in 1 ms
10:27:35,940 INFO  [DefaultSoapUICore] All plugins loaded
java.lang.NullPointerException: Cannot invoke "com.eviware.soapui.impl.InterfaceFactory.build(com.eviware.soapui.impl.wsdl.WsdlProject, com.eviware.soapui.config.InterfaceConfig)" because the return value of "java.util.Map.get(Object)" is null
        at com.eviware.soapui.impl.wsdl.InterfaceFactoryRegistry.build(InterfaceFactoryRegistry.java:51)
        at com.eviware.soapui.impl.wsdl.WsdlProject.loadProjectFromInputStream(WsdlProject.java:388)
        at com.eviware.soapui.impl.wsdl.WsdlProject.loadProject(WsdlProject.java:316)
        at com.eviware.soapui.impl.wsdl.WsdlProject.<init>(WsdlProject.java:219)
        at com.eviware.soapui.impl.wsdl.WsdlProjectFactory.createNew(WsdlProjectFactory.java:41)
        at com.eviware.soapui.impl.wsdl.WsdlProjectFactory.createNew(WsdlProjectFactory.java:28)
        at com.eviware.soapui.tools.SoapUITestCaseRunner.runRunner(SoapUITestCaseRunner.java:349)
        at com.eviware.soapui.tools.AbstractSoapUIRunner.run(AbstractSoapUIRunner.java:202)
        at com.eviware.soapui.maven2.TestMojo.execute(TestMojo.java:114)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
        at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
        at org.apache.maven.cli.MavenCli.execute(MavenCli.java:957)
        at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:289)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:193)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:564)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
10:27:36,647 ERROR [SoapUI] An error occurred [Failed to load project from file [file:/C:/PP/code/RAPORTADO/pro-ject/REST-Project-1-readyapi-project.xml]], see error log for details
com.eviware.soapui.support.SoapUIException: Failed to load project from file [file:/C:/PP/code/RAPORTADO/pro-ject/REST-Project-1-readyapi-project.xml]
        at com.eviware.soapui.impl.wsdl.WsdlProject.loadProject(WsdlProject.java:333)
        at com.eviware.soapui.impl.wsdl.WsdlProject.<init>(WsdlProject.java:219)
        at com.eviware.soapui.impl.wsdl.WsdlProjectFactory.createNew(WsdlProjectFactory.java:41)
        at com.eviware.soapui.impl.wsdl.WsdlProjectFactory.createNew(WsdlProjectFactory.java:28)
        at com.eviware.soapui.tools.SoapUITestCaseRunner.runRunner(SoapUITestCaseRunner.java:349)
        at com.eviware.soapui.tools.AbstractSoapUIRunner.run(AbstractSoapUIRunner.java:202)
        at com.eviware.soapui.maven2.TestMojo.execute(TestMojo.java:114)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:137)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:210)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:156)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:148)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:117)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:81)
        at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:56)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:305)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:192)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:105)
        at org.apache.maven.cli.MavenCli.execute(MavenCli.java:957)
        at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:289)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:193)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:564)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:282)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:225)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:406)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:347)
Caused by: java.lang.NullPointerException: Cannot invoke "com.eviware.soapui.impl.InterfaceFactory.build(com.eviware.soapui.impl.wsdl.WsdlProject, com.eviware.soapui.config.InterfaceConfig)" because the return value of "java.util.Map.get(Object)" is null
        at com.eviware.soapui.impl.wsdl.InterfaceFactoryRegistry.build(InterfaceFactoryRegistry.java:51)
        at com.eviware.soapui.impl.wsdl.WsdlProject.loadProjectFromInputStream(WsdlProject.java:388)
        at com.eviware.soapui.impl.wsdl.WsdlProject.loadProject(WsdlProject.java:316)
        ... 28 more
[ERROR] java.lang.Exception: Failed to load SoapUI project file [REST-Project-1-readyapi-project.xml]

However, as soon as I remove the entire <con:interface> sub-tree from the project XML, the runner proceeds to load and run it (it obviously fails, but my point remains).

Known bug? Stupid mistake?