diff --git a/gc.c b/gc.c
index d8ff277..b789a24 100644
--- a/gc.c
+++ b/gc.c
@@ -403,8 +403,8 @@ static value_t _get_hash_value(value_t v, seen_value_t *seen)
   {
     /* Lists and trees compare by value, not reference. */
     value_t seed = fixnum_value(0);
-    seed = combine_hash_values(seed, _get_hash_value(_CAR(v)), &new_seen);
-    seed = combine_hash_values(seed, _get_hash_value(_CDR(v)), &new_seen);
+    seed = combine_hash_values(seed, _get_hash_value(_CAR(v), &new_seen));
+    seed = combine_hash_values(seed, _get_hash_value(_CDR(v), &new_seen));
     return seed;
   }
   else if (is_box(v) || is_weak_box(v))