--- a/hgext3rd/topic/__init__.py	Tue Aug 29 19:03:59 2017 +0530
+++ b/hgext3rd/topic/__init__.py	Tue Aug 29 19:12:00 2017 +0530
@@ -73,6 +73,7 @@
     patch,
     phases,
     registrar,
+    scmutil,
     templatefilters,
     util,
 )
@@ -315,7 +316,7 @@
         ('', 'current', None, 'display the current topic only'),
     ] + commands.formatteropts,
     _('hg topics [TOPIC]'))
-def topics(ui, repo, topic='', clear=False, rev=None, list=False, **opts):
+def topics(ui, repo, topic=None, clear=False, rev=None, list=False, **opts):
     """View current topic, set current topic, change topic for a set of revisions, or see all topics.
 
     Clear topic on existing topiced revisions:
@@ -353,6 +354,13 @@
     if clear and topic:
         raise error.Abort(_("cannot use --clear when setting a topic"))
 
+    if topic:
+        topic = topic.strip()
+        if not topic:
+            raise error.Abort(_("topic name cannot consist entirely of whitespaces"))
+        # Have some restrictions on the topic name just like bookmark name
+        scmutil.checknewlabel(repo, topic, 'topic')
+
     if list:
         if clear or rev:
             raise error.Abort(_("cannot use --clear or --rev with --list"))

--- a/tests/test-topic.t	Tue Aug 29 19:03:59 2017 +0530
+++ b/tests/test-topic.t	Tue Aug 29 19:12:00 2017 +0530
@@ -93,10 +93,20 @@
 Trying some invalid topicnames
 
   $ hg topic '.'
+  abort: the name '.' is reserved
+  [255]
   $ hg topic null
+  abort: the name 'null' is reserved
+  [255]
   $ hg topic tip
+  abort: the name 'tip' is reserved
+  [255]
   $ hg topic 12345
+  abort: cannot use an integer as a name
+  [255]
   $ hg topic '   '
+  abort: topic name cannot consist entirely of whitespaces
+  [255]
 
 Test commit flag and help text