SVN Externals, do not work when pointed to a specific revision number

007-08-14 13:49:05,304 INFO [BAM::BuildChangeDetector] [SvnRepository] Checking externals for DEF
2007-08-14 13:49:05,882 INFO [BAM::BuildChangeDetector] [SvnRepository] Externals revision for externals/test: LOCAL: 52 HEAD: 72
2007-08-14 13:49:08,445 INFO [BAM::BuildChangeDetector] [SvnRepository] Updating Subversion to pick up any svn:externals changes
2007-08-14 13:49:08,742 INFO [BAM::BuildChangeDetector] [UpdateEventHandler]
2007-08-14 13:49:09,179 INFO [BAM::BuildChangeDetector] [UpdateEventHandler] Fetching external item into 'D:\bamboo\bamboo-home\xml-data\build-dir\PR-DEF\externals\test'
2007-08-14 13:49:09,179 INFO [BAM::BuildChangeDetector] [UpdateEventHandler] External at revision -1
2007-08-14 13:49:09,257 INFO [BAM::BuildChangeDetector] [UpdateEventHandler] externals/test
2007-08-14 13:49:09,273 INFO [BAM::BuildChangeDetector] [UpdateEventHandler] At revision 52

2007-08-14 13:49:12,101 INFO [BAM::BuildChangeDetector] [DefaultErrorHandler] Recording error: Failed to get the build source code : PR - Default
com.atlassian.bamboo.repository.RepositoryException: PR - Default
at com.atlassian.bamboo.repository.svn.SvnRepository.getChangesSinceLastBuild(SvnRepository.java:459)
at com.atlassian.bamboo.build.strategy.PollingBuildStrategy.requiresBuild(PollingBuildStrategy.java:128)
at com.atlassian.bamboo.build.DefaultBuildChangeDetector.checkIfBuildNeedsBuilding(DefaultBuildChangeDetector.java:170)
at com.atlassian.bamboo.build.DefaultBuildChangeDetector.doTask(DefaultBuildChangeDetector.java:118)
at com.atlassian.bamboo.build.DefaultBuildChangeDetector.run(DefaultBuildChangeDetector.java:81)
at com.atlassian.bamboo.build.pipeline.concurrent.NamedThreadFactory$1.run(NamedThreadFactory.java:31)
at edu.emory.mathcs.backport.java.util.concurrent.helpers.ThreadHelpers$1.run(ThreadHelpers.java:34)
at java.lang.Thread.run(Unknown Source)
Caused by: org.tmatesoft.svn.core.SVNException: svn: Externals update failed - local revision does not match expected revision.
at com.atlassian.bamboo.repository.svn.SvnRepository.getChangesSinceLastBuild(SvnRepository.java:422)
... 7 more