diff --git a/spot/tl/sonf.cc b/spot/tl/sonf.cc
index 29b613eaa..0b70ab5e3 100644
--- a/spot/tl/sonf.cc
+++ b/spot/tl/sonf.cc
@@ -131,7 +131,7 @@ namespace spot
           {
             // recurse into rhs first (_ []-> rhs)
             formula rhs =
-              f[1].map(extractor, extracted, extractor, false, false);
+              extractor(f[1], extracted, extractor, false, false);
             f = formula::binop(kind, f[0], rhs);
 
             formula ap = formula::ap(new_ap_name());