--- a/hgsubversion/maps.py
+++ b/hgsubversion/maps.py
@@ -98,7 +98,10 @@ class AuthorMap(dict):
 
 
 class TagMap(dict):
+    """Map tags to converted node identifier.
 
+    tag names are non-empty strings.
+    """
     VERSION = 2
 
     @classmethod
@@ -133,6 +136,8 @@ class TagMap(dict):
             tag = tag[:-1]
             if self.endrev is not None and revision > self.endrev:
                 break
+            if not tag:
+                continue
             dict.__setitem__(self, tag, node.bin(hash))
         f.close()
 
@@ -148,14 +153,17 @@ class TagMap(dict):
             self[k] = v
 
     def __contains__(self, tag):
-        return dict.__contains__(self, tag) and dict.__getitem__(self, tag) != node.nullid
+        return (tag and dict.__contains__(self, tag) 
+                and dict.__getitem__(self, tag) != node.nullid)
 
     def __getitem__(self, tag):
-        if tag in self:
+        if tag and tag in self:
             return dict.__getitem__(self, tag)
         raise KeyError()
 
     def __setitem__(self, tag, info):
+        if not tag:
+            raise hgutil.Abort('tag cannot be empty')
         hash, revision = info
         f = open(self.path, 'a')
         f.write('%s %s %s\n' % (node.hex(hash), revision, tag))

--- a/hgsubversion/svnmeta.py
+++ b/hgsubversion/svnmeta.py
@@ -300,7 +300,7 @@ class SVNMeta(object):
             src_tag = self.get_path_tag(src_path)
             if src_tag or src_file == '':
                 ln = self.localname(p)
-                if src_tag and src_tag in self.tags:
+                if src_tag in self.tags:
                     ci = self.repo[self.tags[src_tag]].extra()['convert_revision']
                     src_rev, src_branch, = self.parse_converted_revision(ci)
                 return {ln: (src_branch, src_rev, revnum)}
@@ -350,7 +350,7 @@ class SVNMeta(object):
         '''
         tag = self.get_path_tag(self.remotename(branch))
         limitedtags = maps.TagMap(self.repo, endrev=number-1)
-        if tag and tag in limitedtags:
+        if tag in limitedtags:
             ha = limitedtags[tag]
             return ha
         r, br = self.get_parent_svn_branch_and_rev(number, branch)