--- a/hgsubversion/maps.py
+++ b/hgsubversion/maps.py
@@ -339,7 +339,13 @@ class BranchMap(dict):
             src = src.strip()
             dst = dst.strip()
             self.ui.debug('adding branch %s to branch map\n' % src)
-            if src in self and dst != self[src]:
+
+            if not dst:
+                # prevent people from assuming such lines are valid
+                raise hgutil.Abort('removing branches is not supported, yet\n'
+                                   '(line %i in branch map %s)'
+                                   % (number, path))
+            elif src in self and dst != self[src]:
                 msg = 'overriding branch: "%s" to "%s" (%s)\n'
                 self.ui.status(msg % (self[src], dst, src))
             self[src] = dst