Index: contrib/client-side/svnmerge.py
===================================================================
--- contrib/client-side/svnmerge.py	(revision 19305)
+++ contrib/client-side/svnmerge.py	(working copy)
@@ -765,8 +765,12 @@
         del props[directory]
 
     if len(props) > 1:
-        error('multiple heads found. '
-              'Explicit head argument (-S/--head) required.')
+	err_msg  = 'multiple heads found. '
+	err_msg += 'Explicit head argument (-S/--head) required.\n'
+	err_msg += 'The head values available(relative to the repos root) are:'
+        for prop in props:
+	  err_msg += '\n' + prop
+        error(err_msg)
 
     return props.keys()[0]