diff --git a/offlineimap/init.py b/offlineimap/init.py
index 3aaa679..7258365 100644
--- a/offlineimap/init.py
+++ b/offlineimap/init.py
@@ -218,7 +218,7 @@ class OfflineImap:
                 options.debugtype = 'imap,maildir,thread'
             #force single threading?
             if not ('thread' in options.debugtype.split(',') \
-                    and options.singlethreading):
+                    and not options.singlethreading):
                 ui._msg("Debug mode: Forcing to singlethreaded.")
                 options.singlethreading = True