diff --git a/src/calibre/gui2/dialogs/tag_categories.py b/src/calibre/gui2/dialogs/tag_categories.py
index af6632bb02..9bddb817cf 100644
--- a/src/calibre/gui2/dialogs/tag_categories.py
+++ b/src/calibre/gui2/dialogs/tag_categories.py
@@ -171,6 +171,13 @@ class TagCategories(QDialog, Ui_TagCategories):
         cat_name = unicode(self.input_box.text()).strip()
         if cat_name == '':
             return False
+        comps = [c.strip() for c in cat_name.split('.') if c.strip()]
+        if len(comps) == 0 or '.'.join(comps) != cat_name:
+            error_dialog(self, _('Invalid name'),
+                    _('That name contains leading or trailing periods, '
+                      'multiple periods in a row or spaces before '
+                      'or after periods.')).exec_()
+            return False
         for c in self.categories:
             if strcmp(c, cat_name) == 0:
                 error_dialog(self, _('Name already used'),
@@ -193,6 +200,14 @@ class TagCategories(QDialog, Ui_TagCategories):
             return False
         if not self.current_cat_name:
             return False
+        comps = [c.strip() for c in cat_name.split('.') if c.strip()]
+        if len(comps) == 0 or '.'.join(comps) != cat_name:
+            error_dialog(self, _('Invalid name'),
+                    _('That name contains leading or trailing periods, '
+                      'multiple periods in a row or spaces before '
+                      'or after periods.')).exec_()
+            return False
+
         for c in self.categories:
             if strcmp(c, cat_name) == 0:
                 error_dialog(self, _('Name already used'),
diff --git a/src/calibre/gui2/tag_view.py b/src/calibre/gui2/tag_view.py
index 83ea97b880..e4b4552504 100644
--- a/src/calibre/gui2/tag_view.py
+++ b/src/calibre/gui2/tag_view.py
@@ -685,7 +685,9 @@ class TagsModel(QAbstractItemModel): # {{{
         self.filter_categories_by = filter_categories_by
         self.collapse_model = collapse_model
 
-        # get_node_tree cannot return None here, because row_map is empty
+        # get_node_tree cannot return None here, because row_map is empty. Note
+        # that get_node_tree can indirectly change the user_categories dict.
+
         data = self.get_node_tree(config['sort_tags_by'])
         gst = db.prefs.get('grouped_search_terms', {})
         self.root_item = TagTreeItem()
@@ -707,7 +709,7 @@ class TagsModel(QAbstractItemModel): # {{{
                 tt = _(u'The lookup/search name is "{0}"').format(r)
 
             if r.startswith('@'):
-                path_parts = [p.strip() for p in r.split('.') if p.strip()]
+                path_parts = [p for p in r.split('.')]
                 path = ''
                 last_category_node = self.root_item
                 tree_root = self.category_node_tree
@@ -1178,7 +1180,10 @@ class TagsModel(QAbstractItemModel): # {{{
                 # category display order is important here. The following works
                 # only of all the non-user categories are displayed before the
                 # user categories
-                components = [t for t in original_name(tag).split('.')]
+                components = [t.strip() for t in original_name(tag).split('.')
+                              if t.strip()]
+                if len(components) == 0 or '.'.join(components) != original_name(tag):
+                    components = [original_name(tag)]
                 in_uc = fm['kind'] == 'user'
                 if (not tag.is_hierarchical) and (in_uc or
                         key in ['authors', 'publisher', 'news', 'formats', 'rating'] or
@@ -1264,7 +1269,7 @@ class TagsModel(QAbstractItemModel): # {{{
         # working with the last item and that item is deleted, in which case
         # we position at the parent label
         path = index.model().path_for_index(index)
-        val = unicode(value.toString())
+        val = unicode(value.toString()).strip()
         if not val:
             error_dialog(self.tags_view, _('Item is blank'),
                         _('An item cannot be set to nothing. Delete it instead.')).exec_()
diff --git a/src/calibre/library/database2.py b/src/calibre/library/database2.py
index 5155afe7e9..4be2ba4340 100644
--- a/src/calibre/library/database2.py
+++ b/src/calibre/library/database2.py
@@ -1181,6 +1181,22 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             return 'n=%s s=%s c=%d rt=%d rc=%d id=%s'%\
                             (self.n, self.s, self.c, self.rt, self.rc, self.id)
 
+    def clean_user_categories(self):
+        user_cats = self.prefs.get('user_categories', {})
+        new_cats = {}
+        for k in user_cats:
+            comps = [c.strip() for c in k.split('.') if c.strip()]
+            if len(comps) == 0:
+                i = 1
+                while True:
+                    if unicode(i) not in user_cats:
+                        new_cats[unicode(i)] = user_cats[k]
+                        break
+                    i += 1
+            else:
+                new_cats['.'.join(comps)] = user_cats[k]
+        self.prefs.set('user_categories', new_cats)
+        return new_cats
 
     def get_categories(self, sort='name', ids=None, icon_map=None):
         #start = last = time.clock()
@@ -1421,7 +1437,7 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             categories['formats'].sort(key = lambda x:x.name)
 
         #### Now do the user-defined categories. ####
-        user_categories = dict.copy(self.prefs['user_categories'])
+        user_categories = dict.copy(self.clean_user_categories())
 
         # We want to use same node in the user category as in the source
         # category. To do that, we need to find the original Tag node. There is