diff --git a/src/Defun.ml b/src/Defun.ml
index d464136..e92b7e3 100644
--- a/src/Defun.ml
+++ b/src/Defun.ml
@@ -77,7 +77,11 @@ let rec walk_term fs t =
                 vars
                 (S.vvars (List.tl args)) in
             let thisTag = freshTag () in
-            let freeVars = Atom.Set.elements @@ S.fv_term body in
+            let freeVars = Atom.Set.elements @@
+                Atom.Set.diff
+                    (S.fv_term body)
+                    (Atom.Set.of_list vars)
+                in
             let thisFunc =
                 T.Branch(thisTag, freeVars, nBody) in
             let fs = add_func fs arity thisFunc in