diff --git a/wrap/python/ajax/css/ltl2tgba.css b/wrap/python/ajax/css/ltl2tgba.css
index ed3e51bd1..51d5b6951 100644
--- a/wrap/python/ajax/css/ltl2tgba.css
+++ b/wrap/python/ajax/css/ltl2tgba.css
@@ -80,6 +80,10 @@ div.ltl2tgba {
     font-size: 1.1em;
 }
 
+.ltl2tgba .error {
+    color: red;
+}
+
 .ltl2tgba .parse-error {
     font-family: monospace;
     white-space: pre;
diff --git a/wrap/python/ajax/spot.in b/wrap/python/ajax/spot.in
index 67db4b7b6..93bddd601 100755
--- a/wrap/python/ajax/spot.in
+++ b/wrap/python/ajax/spot.in
@@ -369,6 +369,12 @@ if output_type == 'f':
 # Formula translation.
 translator = form.getfirst('t', 'fm')
 
+if f.is_psl_formula() and not f.is_ltl_formula() and translator != 'fm':
+    print ('''<div class="error">The PSL formula
+<span class="formula spot-format">%s</span>
+cannot be translated using this algorithm.  Please use Couveur/FM.''' % f);
+    finish()
+
 dict = spot.bdd_dict()
 
 if translator == 'fm':