--- a/hgext3rd/topic/destination.py	Mon Apr 04 00:28:03 2016 -0700
+++ b/hgext3rd/topic/destination.py	Wed May 11 16:06:58 2016 +0200
@@ -9,7 +9,8 @@
     util,
 )
 
-def _destmergebranch(orig, repo, action='merge', sourceset=None, onheadcheck=True):
+def _destmergebranch(orig, repo, action='merge', sourceset=None,
+                     onheadcheck=True, destspace=None):
     p1 = repo['.']
     top = p1.topic()
     if top:
@@ -42,8 +43,10 @@
                               % (top, len(heads)))
         else:
             assert False # that's impossible
-    if getattr(orig, 'func_default', ()): # version above hg-3.7
+    if len(getattr(orig, 'func_defaults', ())) == 3: # version hg-3.7
         return orig(repo, action, sourceset, onheadcheck)
+    if 3 < len(getattr(orig, 'func_defaults', ())): # version hg-3.8 and above
+        return orig(repo, action, sourceset, onheadcheck, destspace=destspace)
     else:
         return orig(repo)