diff --git a/NEWS b/NEWS
index c5cff5310..42ad55dbc 100644
--- a/NEWS
+++ b/NEWS
@@ -30,6 +30,12 @@ New in spot 2.3.1.dev (not yet released)
     anymore and should be replaced by the newly-introduced --format=%x
     for consistency with tools producing automata.
 
+  Backward-incompatible changes:
+
+  - spot::acc_cond::mark_t::operator bool() has been marked as
+    explicit.  The implicit converion to bool (and, via bool, to int)
+    was a source of bugs.
+
 
 New in spot 2.3.1 (2017-02-20)
 
diff --git a/spot/twa/acc.hh b/spot/twa/acc.hh
index 758ac476f..197804d3d 100644
--- a/spot/twa/acc.hh
+++ b/spot/twa/acc.hh
@@ -98,7 +98,7 @@ namespace spot
         return id >= o.id;
       }
 
-      operator bool() const
+      explicit operator bool() const
       {
         return id != 0;
       }
diff --git a/spot/twaalgos/remfin.cc b/spot/twaalgos/remfin.cc
index 87d8c90ef..d25c26205 100644
--- a/spot/twaalgos/remfin.cc
+++ b/spot/twaalgos/remfin.cc
@@ -260,7 +260,7 @@ namespace spot
               i -= f;
               for (auto s: states)
                 {
-                  bool acc = aut->state_acc_sets(s) & i;
+                  bool acc{aut->state_acc_sets(s) & i};
                   for (auto& t: aut->out(s))
                     res->new_acc_edge(s, t.dst, t.cond, acc);
                 }
@@ -282,7 +282,7 @@ namespace spot
                       acc_cond::mark_t acc = aut->state_acc_sets(s);
                       if (acc.has(r))
                         continue;
-                      bool jacc = acc & inf_alone;
+                      bool jacc{acc & inf_alone};
                       bool cacc = fin_alone.has(r) || acc.has(r + 1);
                       for (auto& t: aut->out(s))
                         {