diff --git a/eval.c b/eval.c
index b58012af..4009a4f2 100644
--- a/eval.c
+++ b/eval.c
@@ -141,7 +141,7 @@ sexp sexp_env_cell_define (sexp ctx, sexp env, sexp key,
     }
 #endif
   for (ls=sexp_env_bindings(env); sexp_pairp(ls); ls=sexp_env_next_cell(ls))
-    if (sexp_car(ls) == key && sexp_cdr(ls) == SEXP_UNDEF) {
+    if (sexp_car(ls) == key) {
       sexp_cdr(ls) = value;
       return ls;
     }