diff --git a/ChangeLog b/ChangeLog
index dbdfd48ad..39afdb2c5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2010-04-15  Alexandre Duret-Lutz  <adl@lrde.epita.fr>
+
+	Fix simplification of "a M true" as Fa.
+
+	* src/ltlvisit/simpfg.cc: Typo.
+	* src/ltltest/reduccmp.test: Add more tests.
+
 2010-04-15  Alexandre Duret-Lutz  <adl@lrde.epita.fr>
 
 	* HACKING: Bison 2.4.2 has a bugfix we rely on.
diff --git a/src/ltltest/reduccmp.test b/src/ltltest/reduccmp.test
index b35e89258..62a5374e2 100755
--- a/src/ltltest/reduccmp.test
+++ b/src/ltltest/reduccmp.test
@@ -66,6 +66,10 @@ for x in ../reduccmp ../reductaustr; do
   run 0 $x 'a | (b U a) | a' '(b U a)'
   run 0 $x 'a U (b U a)' '(b U a)'
 
+  run 0 $x 'a M 1' 'Fa'
+  run 0 $x 'a W 0' 'Ga'
+  run 0 $x '1 U a' 'Fa'
+  run 0 $x '0 R a' 'Ga'
 
   # Basics reduction
   run 0 $x 'X(true)' 'true'
diff --git a/src/ltlvisit/simpfg.cc b/src/ltlvisit/simpfg.cc
index f91d58c6c..f4a0f9210 100644
--- a/src/ltlvisit/simpfg.cc
+++ b/src/ltlvisit/simpfg.cc
@@ -78,8 +78,8 @@ namespace spot
 	  return;
 	  /* f1 M true == F(f1) */
 	case binop::M:
-	  if (f1 == constant::true_instance())
-	    result_ = unop::instance(unop::F, f2);
+	  if (f2 == constant::true_instance())
+	    result_ = unop::instance(unop::F, f1);
 	  else
 	    result_ = binop::instance(binop::M, f1, f2);
 	  return;