diff --git a/src/backends/packrat.c b/src/backends/packrat.c
index 25a0966..c67c3e4 100644
--- a/src/backends/packrat.c
+++ b/src/backends/packrat.c
@@ -103,7 +103,7 @@ HParseResult* grow(HParserCacheKey *k, HParseState *state, HRecursionHead *head)
   HParseResult *old_res = old_cached->right->result;
   
   // reset the eval_set of the head of the recursion at each beginning of growth
-  head->eval_set = head->involved_set;  // BUG: this must be a copy
+  head->eval_set = h_slist_copy(head->involved_set);
   HParseResult *tmp_res = perform_lowlevel_parse(state, k->parser);
 
   if (tmp_res) {
diff --git a/src/datastructures.c b/src/datastructures.c
index b1e4f75..3d94804 100644
--- a/src/datastructures.c
+++ b/src/datastructures.c
@@ -41,6 +41,26 @@ HSlist* h_slist_new(HArena *arena) {
   return ret;
 }
 
+HSlist* h_slist_copy(HSlist *slist) {
+  HSlist *ret = h_slist_new(slist->arena);
+  HSlistNode *head = slist->head;
+  HSlistNode *tail;
+  if (head != NULL) {
+    h_slist_push(ret, head->elem);
+    tail = ret->head;
+    head = head->next;
+  }
+  while (head != NULL) {
+    // append head item to tail in a new node
+    HSlistNode *node = h_arena_malloc(slist->arena, sizeof(HSlistNode));
+    node->elem = head->elem;
+    node->next = NULL;
+    tail = tail->next = node;
+    head = head->next;
+  }
+  return ret;
+}
+
 void* h_slist_pop(HSlist *slist) {
   HSlistNode *head = slist->head;
   if (!head)
diff --git a/src/internal.h b/src/internal.h
index 67ecb22..0dcf857 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -209,6 +209,7 @@ HCountedArray *h_carray_new(HArena * arena);
 void h_carray_append(HCountedArray *array, void* item);
 
 HSlist* h_slist_new(HArena *arena);
+HSlist* h_slist_copy(HSlist *slist);
 void* h_slist_pop(HSlist *slist);
 void h_slist_push(HSlist *slist, void* item);
 bool h_slist_find(HSlist *slist, const void* item);