diff --git a/c/tskit/tables.c b/c/tskit/tables.c
index 3dff91728e..cc0345a977 100644
--- a/c/tskit/tables.c
+++ b/c/tskit/tables.c
@@ -12893,22 +12893,31 @@ typedef struct _edge_list_t {
     struct _edge_list_t *next;
 } edge_list_t;
 
-static edge_list_t *TSK_WARN_UNUSED
-extend_edges_alloc_entry(tsk_blkalloc_t *heap, tsk_id_t edge)
+static int
+extend_edges_append_entry(
+    edge_list_t **head, edge_list_t **tail, tsk_blkalloc_t *heap, tsk_id_t edge)
 {
+    int ret = 0;
     edge_list_t *x = NULL;
 
     x = tsk_blkalloc_get(heap, sizeof(*x));
     if (x == NULL) {
+        ret = TSK_ERR_NO_MEMORY;
         goto out;
     }
-    tsk_bug_assert(edge >= 0);
 
     x->edge = edge;
     x->extended = false;
     x->next = NULL;
+
+    if (*tail == NULL) {
+        *head = x;
+    } else {
+        (*tail)->next = x;
+    }
+    *tail = x;
 out:
-    return x;
+    return ret;
 }
 
 static void
@@ -12953,7 +12962,7 @@ forward_extend(tsk_table_collection_t *self, int direction)
     tsk_blkalloc_t edge_list_heap;
     edge_list_t *edges_in_head, *edges_in_tail;
     edge_list_t *edges_out_head, *edges_out_tail;
-    edge_list_t *x, *y, *ex1, *ex2, *ex_in;
+    edge_list_t *ex1, *ex2, *ex_in;
     tsk_edge_table_t edges;
     tsk_edge_t edge;
     double sign, here, there;
@@ -13021,36 +13030,24 @@ forward_extend(tsk_table_collection_t *self, int direction)
 
         while (((tk < M) && (tk >= 0)) && (far_side[O[tk]] == here)) {
             // add edge tk to pending_out
-            x = extend_edges_alloc_entry(&edge_list_heap, O[tk]);
-            if (x == NULL) {
+            ret = extend_edges_append_entry(
+                &edges_out_head, &edges_out_tail, &edge_list_heap, O[tk]);
+            if (ret != 0) {
                 ret = TSK_ERR_NO_MEMORY;
                 goto out;
             }
-            if (edges_out_tail == NULL) {
-                edges_out_head = x;
-            } else {
-                y = edges_out_tail;
-                y->next = x;
-            }
-            edges_out_tail = x;
             num_children[edges.parent[O[tk]]] -= 1;
             num_children[edges.child[O[tk]]] -= 1;
             tk += sign_int;
         }
         while (((tj < M) && (tj >= 0)) && (near_side[I[tj]] == here)) {
             // add edge tj to pending_in
-            x = extend_edges_alloc_entry(&edge_list_heap, I[tj]);
-            if (x == NULL) {
+            ret = extend_edges_append_entry(
+                &edges_in_head, &edges_in_tail, &edge_list_heap, I[tj]);
+            if (ret != 0) {
                 ret = TSK_ERR_NO_MEMORY;
                 goto out;
             }
-            if (edges_in_tail == NULL) {
-                edges_in_head = x;
-            } else {
-                y = edges_in_tail;
-                y->next = x;
-            }
-            edges_in_tail = x;
             num_children[edges.parent[I[tj]]] += 1;
             num_children[edges.child[I[tj]]] += 1;
             tj += sign_int;