mysterious 'string index out of bounds' exception during git merge

[ChainExecutionManagerImpl] Exception during merge: String index out of range: 65
java.lang.StringIndexOutOfBoundsException: String index out of range: 65
        at java.lang.String.<init>(String.java:211)
        at java.lang.StringBuilder.toString(StringBuilder.java:430)
        at com.atlassian.bamboo.plugins.git.GitCommandProcessor$LoggingOutputHandler.getStdout(GitCommandProcessor.java:623)
        at com.atlassian.bamboo.plugins.git.GitCommandProcessor.runCommand(GitCommandProcessor.java:493)
        at com.atlassian.bamboo.plugins.git.GitCommandProcessor.runMergeCommand(GitCommandProcessor.java:512)
        at com.atlassian.bamboo.plugins.git.NativeGitOperationHelper.merge(NativeGitOperationHelper.java:240)
        at com.atlassian.bamboo.plugins.git.GitRepository.mergeWorkspaceWith(GitRepository.java:520)
        at com.atlassian.bamboo.plan.branch.BranchIntegrationHelperImpl.merge(BranchIntegrationHelperImpl.java:114)
        at com.atlassian.bamboo.chains.ChainExecutionManagerImpl.doVcsMerge(ChainExecutionManagerImpl.java:951)
        at com.atlassian.bamboo.chains.ChainExecutionManagerImpl.access$1300(ChainExecutionManagerImpl.java:97)
        at com.atlassian.bamboo.chains.ChainExecutionManagerImpl$5.run(ChainExecutionManagerImpl.java:601)
        at com.atlassian.util.concurrent.ManagedLocks$ManagedLockImpl.withLock(ManagedLocks.java:321)
        at com.atlassian.bamboo.chains.ChainExecutionManagerImpl.execute(ChainExecutionManagerImpl.java:584)
        at com.atlassian.bamboo.chains.ChainExecutionManagerImpl.tryStartChainState(ChainExecutionManagerImpl.java:288)
        at com.atlassian.bamboo.chains.ChainExecutionManagerImpl.start(ChainExecutionManagerImpl.java:171)
        at sun.reflect.GeneratedMethodAccessor1133.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:304)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)
        at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
        at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
        at com.sun.proxy.$Proxy101.start(Unknown Source)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3$2.call(PlanExecutionManagerImpl.java:476)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3$2.call(PlanExecutionManagerImpl.java:469)
        at com.atlassian.util.concurrent.ManagedLocks$ManagedLockImpl.withLock(ManagedLocks.java:312)
        at com.atlassian.bamboo.plan.PlanExecutionLockServiceImpl.inlineProcessLocks(PlanExecutionLockServiceImpl.java:99)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.doWithInlineProcessLock(PlanExecutionManagerImpl.java:680)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.access$400(PlanExecutionManagerImpl.java:123)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3.call(PlanExecutionManagerImpl.java:468)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3.call(PlanExecutionManagerImpl.java:449)
        at com.atlassian.util.concurrent.ManagedLocks$ManagedLockImpl.withLock(ManagedLocks.java:312)
        at com.atlassian.bamboo.plan.PlanExecutionLockServiceImpl.lock(PlanExecutionLockServiceImpl.java:81)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.doWithProcessLock(PlanExecutionManagerImpl.java:725)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.startConditionalBuild(PlanExecutionManagerImpl.java:448)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.start(PlanExecutionManagerImpl.java:519)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.start(PlanExecutionManagerImpl.java:536)
        at com.atlassian.bamboo.plan.NonBlockingPlanExecutionServiceImpl$3.call(NonBlockingPlanExecutionServiceImpl.java:137)
        at com.atlassian.bamboo.plan.NonBlockingPlanExecutionServiceImpl$3.call(NonBlockingPlanExecutionServiceImpl.java:124)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
        at com.atlassian.bamboo.build.pipeline.concurrent.NamedThreadFactory$2.run(NamedThreadFactory.java:55)
        at java.lang.Thread.run(Thread.java:680)
:        at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:304)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)
        at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89)
        at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
        at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
        at com.sun.proxy.$Proxy101.start(Unknown Source)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3$2.call(PlanExecutionManagerImpl.java:476)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3$2.call(PlanExecutionManagerImpl.java:469)
        at com.atlassian.util.concurrent.ManagedLocks$ManagedLockImpl.withLock(ManagedLocks.java:312)
        at com.atlassian.bamboo.plan.PlanExecutionLockServiceImpl.inlineProcessLocks(PlanExecutionLockServiceImpl.java:99)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.doWithInlineProcessLock(PlanExecutionManagerImpl.java:680)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.access$400(PlanExecutionManagerImpl.java:123)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3.call(PlanExecutionManagerImpl.java:468)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl$3.call(PlanExecutionManagerImpl.java:449)
        at com.atlassian.util.concurrent.ManagedLocks$ManagedLockImpl.withLock(ManagedLocks.java:312)
        at com.atlassian.bamboo.plan.PlanExecutionLockServiceImpl.lock(PlanExecutionLockServiceImpl.java:81)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.doWithProcessLock(PlanExecutionManagerImpl.java:725)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.startConditionalBuild(PlanExecutionManagerImpl.java:448)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.start(PlanExecutionManagerImpl.java:519)
        at com.atlassian.bamboo.plan.PlanExecutionManagerImpl.start(PlanExecutionManagerImpl.java:536)
        at com.atlassian.bamboo.plan.NonBlockingPlanExecutionServiceImpl$3.call(NonBlockingPlanExecutionServiceImpl.java:137)
        at com.atlassian.bamboo.plan.NonBlockingPlanExecutionServiceImpl$3.call(NonBlockingPlanExecutionServiceImpl.java:124)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
        at com.atlassian.bamboo.build.pipeline.concurrent.NamedThreadFactory$2.run(NamedThreadFactory.java:55)
        at java.lang.Thread.run(Thread.java:680)