When I set Repository URL as variable instead of direct url(please see attachment), polling is not working. Checkout and update are working fine.

Apr 10, 2014 10:03:17 AM hudson.scm.SubversionSCM$DescriptorImpl doCheckRevisionPropertiesSupported
INFO: Failed to access subversion repository ${SVN_REPO}
org.tmatesoft.svn.core.SVNException: svn: E125002: Malformed URL '${SVN_REPO}'
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:64)
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:51)
at org.tmatesoft.svn.core.SVNURL.<init>(SVNURL.java:233)
at org.tmatesoft.svn.core.SVNURL.parseURIDecoded(SVNURL.java:115)
at hudson.scm.SubversionSCM$DescriptorImpl.doCheckRevisionPropertiesSupported(SubversionSCM.java:2185)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:298)
–
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:1011)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:644)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:668)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
Caused by: svn: E125002: Malformed URL '${SVN_REPO}'
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:171)
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:126)
at org.tmatesoft.svn.core.SVNURL.<init>(SVNURL.java:232)
... 94 more