diff --git a/src/hoaparse/hoaparse.yy b/src/hoaparse/hoaparse.yy
index f1e8f185f..86ea5f511 100644
--- a/src/hoaparse/hoaparse.yy
+++ b/src/hoaparse/hoaparse.yy
@@ -1415,8 +1415,17 @@ lbtt-guard: STRING
 	      }
 	    else
 	      {
-		res.cur_label =
-		  formula_to_bdd(f, res.h->aut->get_dict(), res.h->aut);
+		if (!f->is_boolean())
+		  {
+		    error(@$,
+			  "non-Boolean transition label (replaced by true)");
+		    res.cur_label = bddtrue;
+		  }
+		else
+		  {
+		    res.cur_label =
+		      formula_to_bdd(f, res.h->aut->get_dict(), res.h->aut);
+		  }
 		f->destroy();
 	      }
 	    delete $1;
diff --git a/src/tests/hoaparse.test b/src/tests/hoaparse.test
index f1a010e32..f9d39d818 100755
--- a/src/tests/hoaparse.test
+++ b/src/tests/hoaparse.test
@@ -86,6 +86,17 @@ State: 1 {0}
 1 0
 2 0 -1 t
 -1
+/* Another automaton.  Here we make sure that guard must
+   be Boolean, even though we the LBT parser we use
+   would accept LTL. */
+2 1t
+0 1
+1 -1 G "b"
+0 -1 & "a" ! "b"
+-1
+1 0
+0 0 -1 t
+-1
 EOF
 
 diff='different state numbers have been used'
@@ -97,6 +108,7 @@ input:2.1-9: ... declared here.
 input:10.5: state number is larger than state count...
 input:2.1-9: ... declared here.
 input:12.1-19.2: 2 states have been declared, but 3 $diff
+input:25.5-10: non-Boolean transition label (replaced by true)
 EOF
 
 cat >input <<EOF