diff --git a/src/ltlast/unop.cc b/src/ltlast/unop.cc
index 04a8d9a40..d8ac2977c 100644
--- a/src/ltlast/unop.cc
+++ b/src/ltlast/unop.cc
@@ -131,9 +131,8 @@ namespace spot
 	    if (child == constant::false_instance()
 		|| child == constant::true_instance())
 	      return child;
-	    // F([*0]) = G([*0]) = 1
-	    if (child == constant::empty_word_instance())
-	      return constant::true_instance();
+
+	    assert(child != constant::empty_word_instance());
 	  }
 	  break;
 
@@ -185,9 +184,7 @@ namespace spot
 	  if (child == constant::true_instance()
 	      || child == constant::false_instance())
 	    return child;
-	  // X([*0]) = 1
-	  if (child == constant::empty_word_instance())
-	    return constant::true_instance();
+	  assert(child != constant::empty_word_instance());
 	  break;
 
 	case Finish:
diff --git a/src/ltlast/unop.hh b/src/ltlast/unop.hh
index 687ecf47b..a23e4ec45 100644
--- a/src/ltlast/unop.hh
+++ b/src/ltlast/unop.hh
@@ -60,10 +60,10 @@ namespace spot
       ///   - GG(Exp) = G(Exp)
       ///   - F(0) = 0
       ///   - G(0) = 0
+      ///   - X(0) = 0
       ///   - F(1) = 1
       ///   - G(1) = 1
-      ///   - F([*0]) = 1
-      ///   - G([*0]) = 1
+      ///   - X(1) = 1
       ///   - !1 = 0
       ///   - !0 = 1
       ///   - ![*0] = 1[+]     (read below)