diff --git a/NEWS b/NEWS
index 70b7a369d67597ea3df9b93844fcc30e09cf1ae9..25682d60cf1bee933518218c91611450460df0a7 100644
--- a/NEWS
+++ b/NEWS
@@ -23,6 +23,11 @@ New in spot 1.99.7a (not yet released)
     pointers) for some concepts used in Spot.
     See https://spot.lrde.epita.fr/concepts.html
 
+  Bugs:
+
+  * Using ltl2tgba -U would fail to output the unambiguous property
+    (regression introduced in 1.99.7)
+
 New in spot 1.99.7 (2016-01-15)
 
   Command-line tools:
diff --git a/spot/twaalgos/ltl2tgba_fm.cc b/spot/twaalgos/ltl2tgba_fm.cc
index e4fd6014aea24bc9094ed16840539e5c63b9ff90..59290c271da29e6be98bfb07d35397ecfb70c35e 100644
--- a/spot/twaalgos/ltl2tgba_fm.cc
+++ b/spot/twaalgos/ltl2tgba_fm.cc
@@ -2193,7 +2193,7 @@ namespace spot
       }
 
     if (unambiguous)
-      a->prop_unambiguous();
+      a->prop_unambiguous(true);
 
     // Set the following to true to preserve state names.
     a->release_formula_namer(namer, false);
diff --git a/tests/core/unambig.test b/tests/core/unambig.test
index 72184a3f87e96e0ce42f8ee358a5a0fb4c59e042..ea36a8912af10a2b5f16947e4b48bd71e24dbba2 100755
--- a/tests/core/unambig.test
+++ b/tests/core/unambig.test
@@ -1,7 +1,7 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2013, 2015 Laboratoire de Recherche et Developpement
-# de l'Epita
+# Copyright (C) 2013, 2015, 2016 Laboratoire de Recherche et
+# Developpement de l'Epita
 #
 # This file is part of Spot, a model checking library.
 #
@@ -187,6 +187,12 @@ State: 2
 --END--
 EOF
 
+
+# Thus should be marked as unambiguous.  Report from Joachim Klein.
+ltl2tgba --lbt-input -B -U -x wdba-minimize=0 "U p0 & p1 X ! p0" |
+    grep 'properties:.*unambiguous'
+
+
 # Make sure we preserve the "unambiguous" property if it is given.
 run 0 $autfilt -H --is-unambiguous input >output