Fork synchronization fails with IllegalArgumentException

2013-09-06 04:02:38,602 WARN  [refsync:thread-3] username 242x487x1 1p1kl8g 10.60.163.76,10.56.188.18 "POST /scm/proj/repo.git/git-receive-pack HTTP/1.1" c.a.s.i.r.sync.For
kRefSyncListener ~forkuser/repo[172]: Could not synchronize changes from proj/repo[32]
java.lang.IllegalArgumentException: null
        at com.google.common.base.Preconditions.checkArgument(Preconditions.java:72) ~[guava-10.0.1.jar:na]
        at com.atlassian.stash.scm.AbstractCommandBuilder.notBlank(AbstractCommandBuilder.java:433) ~[stash-spi-2.6.2.jar:na]
        at com.atlassian.stash.scm.AbstractCommandBuilder.environment(AbstractCommandBuilder.java:300) ~[stash-spi-2.6.2.jar:na]
        at com.atlassian.stash.scm.AbstractCommandBuilder.environment(AbstractCommandBuilder.java:22) ~[stash-spi-2.6.2.jar:na]
        at com.atlassian.stash.internal.scm.git.AbstractGitCommandBuilder.environment(AbstractGitCommandBuilder.java:104) ~[na:na]
        at com.atlassian.stash.internal.scm.git.AbstractGitCommandBuilder.author(AbstractGitCommandBuilder.java:34) ~[na:na]
        at com.atlassian.stash.internal.scm.git.AbstractGitCommandBuilder.author(AbstractGitCommandBuilder.java:43) ~[na:na]
        at com.atlassian.stash.internal.repository.sync.command.AbstractFetchSynchronizeRefsCommand.fetchBuilder(AbstractFetchSynchronizeRefsCommand.java:36) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.command.IncrementalSynchronizeRefsCommand.fetchRefs(IncrementalSynchronizeRefsCommand.java:149) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.command.IncrementalSynchronizeRefsCommand.synchronize(IncrementalSynchronizeRefsCommand.java:55) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.command.AbstractSynchronizeRefsCommand.call(AbstractSynchronizeRefsCommand.java:44) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.command.AbstractSynchronizeRefsCommand.call(AbstractSynchronizeRefsCommand.java:21) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.DefaultRefSyncService$SynchronizeOperation.doPerform(DefaultRefSyncService.java:541) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.DefaultRefSyncService$AbstractOperation.perform(DefaultRefSyncService.java:332) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.DefaultRefSyncService$AbstractOperation.perform(DefaultRefSyncService.java:319) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.ReentrantRepositoryLock.withLock(ReentrantRepositoryLock.java:31) [stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.ReentrantRepositoryLock.withLock(ReentrantRepositoryLock.java:40) [stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.DefaultRefSyncService.synchronize(DefaultRefSyncService.java:291) ~[stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.ForkRefSyncListener$SynchronizeTask.perform(ForkRefSyncListener.java:291) [stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.ForkRefSyncListener$SynchronizeTask.perform(ForkRefSyncListener.java:220) [stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.ReentrantRepositoryLock.withLock(ReentrantRepositoryLock.java:31) [stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.stash.internal.repository.sync.ForkRefSyncListener$SynchronizeTask.run(ForkRefSyncListener.java:259) [stash-repository-ref-sync-2.6.2_1374813200000.jar:na]
        at com.atlassian.sal.core.executor.ThreadLocalDelegateRunnable.run(ThreadLocalDelegateRunnable.java:38) [sal-core-2.10.3.jar:na]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441) [na:1.6.0_31]
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303) [na:1.6.0_31]
        at java.util.concurrent.FutureTask.run(FutureTask.java:138) [na:1.6.0_31]
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) [na:1.6.0_31]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) [na:1.6.0_31]
        at java.lang.Thread.run(Thread.java:662) [na:1.6.0_31]