ParametersResolvingFailedException when labelling the build

Using 5.1.2. I recently modified a configuration to use an Autoincrementer value instead of a manually-entered environment variable. The build still completes successfully, but I get an email about a labelling error:

jetbrains.buildServer.ParametersResolvingFailedException: Cannot resolve values for some of the specified references: QA-2.1.%env.VERSION_RELEASE%
     at jetbrains.buildServer.vcs.impl.VcsLabeler.getLabelText(VcsLabeler.java:1)
     at jetbrains.buildServer.vcs.impl.VcsLabeler.prepareLabel(VcsLabeler.java:8)
     at jetbrains.buildServer.vcs.impl.VcsLabeler.access$200(VcsLabeler.java:69)
     at jetbrains.buildServer.vcs.impl.VcsLabeler$3.buildFinished(VcsLabeler.java:0)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at jetbrains.buildServer.util.EventDispatcher.dispatch(EventDispatcher.java:112)
     at jetbrains.buildServer.serverSide.ServerSideEventDispatcher.access$000(ServerSideEventDispatcher.java:2)
     at jetbrains.buildServer.serverSide.ServerSideEventDispatcher$1$1.run(ServerSideEventDispatcher.java:1)
     at jetbrains.buildServer.serverSide.impl.auth.SecurityContextImpl$2.run(SecurityContextImpl.java:2)
     at jetbrains.buildServer.serverSide.impl.auth.SecurityContextImpl.runAs(SecurityContextImpl.java:2)
     at jetbrains.buildServer.serverSide.impl.auth.SecurityContextImpl.runAs(SecurityContextImpl.java:6)
     at jetbrains.buildServer.serverSide.impl.auth.SecurityContextImpl.runAsSystem(SecurityContextImpl.java:18)
     at jetbrains.buildServer.serverSide.ServerSideEventDispatcher$1.invoke(ServerSideEventDispatcher.java:1)
     at $Proxy4.buildFinished(Unknown Source)
     at jetbrains.buildServer.serverSide.impl.BuildServerImpl.finishBuild(BuildServerImpl.java:417)
     at jetbrains.buildServer.serverSide.impl.BuildServerImpl$6.run(BuildServerImpl.java:2)
     at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
     at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
     at java.util.concurrent.FutureTask.run(Unknown Source)
     at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
     at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
     at java.lang.Thread.run(Unknown Source)

The %env.VERSION_RELEASE% property used to be configured via a custom build parameter, but now it's specified as %autoinc.releasenum%. Can anyone suggest what I'm doing wrong? Thanks