Error using svn:externals with specific revision

I just created a branch in our code base. We are using externals so I changed the svn:externals to refer to a specific revision, like: third-party/skins/toolkit -r21 http://svn.red-bean.com/repos/skin-maker

Bamboo gives the following error when it tries to build:
com.atlassian.bamboo.repository.RepositoryException: MYBUILD builds - Branch-build-162-rev-395
at com.atlassian.bamboo.repository.svn.SvnRepository.getChangesSinceLastBuild(SvnRepository.java:448)
at com.atlassian.bamboo.build.strategy.PollingBuildStrategy.requiresBuild(PollingBuildStrategy.java:128)
at com.atlassian.bamboo.build.DefaultBuildChangeDetector.checkIfBuildNeedsBuilding(DefaultBuildChangeDetector.java:166)
at com.atlassian.bamboo.build.DefaultBuildChangeDetector.doTask(DefaultBuildChangeDetector.java:114)
at com.atlassian.bamboo.build.DefaultBuildChangeDetector.run(DefaultBuildChangeDetector.java:77)
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:413)
... 6 more