modcluster subsystem fails to start because of 'IllegalArgumentException: Invalid name character at offset'

Issue

• The modcluster subsystem fails to start up with an error like the following:

ERROR [org.jboss.as.controller.management-operation] (ServerService Thread Pool -- 10) WFLYCTL0013: Operation ("add") failed - address: ([("subsystem" => "modcluster")]): java.lang.IllegalArgumentException: Invalid name character at offset 50
    at org.jboss.msc.service.ServiceName.invalidNameCharacter(ServiceName.java:494) [jboss-msc-1.2.7.SP1-redhat-1.jar:1.2.7.SP1-redhat-1]
    at org.jboss.msc.service.ServiceName.parse(ServiceName.java:337) [jboss-msc-1.2.7.SP1-redhat-1.jar:1.2.7.SP1-redhat-1]
    at org.jboss.as.controller.ServiceNameFactory.parseServiceName(ServiceNameFactory.java:46) [wildfly-controller-3.0.10.Final-redhat-1.jar:3.0.10.Final-redhat-1]
    at org.jboss.as.controller.OperationContextImpl.getCapabilityServiceName(OperationContextImpl.java:1659) [wildfly-controller-3.0.10.Final-redhat-1.jar:3.0.10.Final-redhat-1]
    at org.jboss.as.controller.ParallelBootOperationContext.getCapabilityServiceName(ParallelBootOperationContext.java:394) [wildfly-controller-3.0.10.Final-redhat-1.jar:3.0.10.Final-redhat-1]
    at org.jboss.as.clustering.controller.UnaryRequirementServiceNameFactory.getServiceName(UnaryRequirementServiceNameFactory.java:44)
    at org.jboss.as.clustering.controller.UnaryServiceNameFactoryProvider.getServiceName(UnaryServiceNameFactoryProvider.java:39)
    at org.wildfly.extension.mod_cluster.ModClusterConfigurationServiceBuilder.lambda$null$5(ModClusterConfigurationServiceBuilder.java:215)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184) [rt.jar:1.8.0_161]
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) [rt.jar:1.8.0_161]
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382) [rt.jar:1.8.0_161]
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) [rt.jar:1.8.0_161]
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) [rt.jar:1.8.0_161]
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151) [rt.jar:1.8.0_161]
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174) [rt.jar:1.8.0_161]
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) [rt.jar:1.8.0_161]
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418) [rt.jar:1.8.0_161]
    at org.wildfly.extension.mod_cluster.ModClusterConfigurationServiceBuilder.lambda$configure$6(ModClusterConfigurationServiceBuilder.java:215)
    at java.util.Optional.ifPresent(Optional.java:159) [rt.jar:1.8.0_161]
    at org.wildfly.extension.mod_cluster.ModClusterConfigurationServiceBuilder.configure(ModClusterConfigurationServiceBuilder.java:212)
    at org.wildfly.extension.mod_cluster.ModClusterSubsystemAdd.performBoottime(ModClusterSubsystemAdd.java:82)

• This occurs after we set the proxies attribute like through via CLI:

/subsystem=modcluster/mod-cluster-config=configuration:write-attribute(name=proxies,value=[proxy1 proxy2])