diff --git a/bin/ltlsynt.cc b/bin/ltlsynt.cc
index 87805801b..64cbde3fd 100644
--- a/bin/ltlsynt.cc
+++ b/bin/ltlsynt.cc
@@ -592,6 +592,14 @@ namespace
         {
           res = solve_formula(f, input_aps_, unknown_aps(f, input_aps_));
         }
+      else if (output_aps_.empty() && input_aps_.empty())
+        {
+          for (const std::string& ap: unknown_aps(f, input_aps_, &output_aps_))
+            error_at_line(2, 0, filename, linenum,
+                          "one of --ins or --outs should list '%s'",
+                          ap.c_str());
+          res = solve_formula(f, input_aps_, output_aps_);
+        }
       else
         {
           for (const std::string& ap: unknown_aps(f, input_aps_, &output_aps_))
diff --git a/tests/core/ltlsynt.test b/tests/core/ltlsynt.test
index f1f0a458b..d2427ea1a 100644
--- a/tests/core/ltlsynt.test
+++ b/tests/core/ltlsynt.test
@@ -541,4 +541,7 @@ grep "'a' appears both" stderr
 
 ltlsynt --ins=a --outs=c -f 'GFa | FGb | GFc' 2>stderr && :
 test $? -eq 2
-grep "but 'b' is unlisted" stderr
+grep "both.*but 'b' is unlisted" stderr
+ltlsynt -f 'GFa | FGb | GFc' 2>stderr && :
+test $? -eq 2
+grep "one of --ins or --outs" stderr