diff --git a/spec/runtime/stack_spec.cc b/spec/runtime/stack_spec.cc
index 721881d5..05be07e8 100644
--- a/spec/runtime/stack_spec.cc
+++ b/spec/runtime/stack_spec.cc
@@ -275,6 +275,30 @@ describe("Stack", [&]() {
         AssertThat(ts_stack_entry_next_count(next), Equals(2));
       });
     });
+
+    describe("when the first head is only one node deep", [&]() {
+      it("adds it as an additional successor node to The Null node", [&]() {
+        /*
+         *    .__A0.
+         *  B1.__/
+         */
+        ts_stack_clear(stack);
+        ts_stack_split(stack, 0);
+        ts_stack_push(stack, 0, stateA, trees[0]);
+        bool merged = ts_stack_push(stack, 1, stateB, trees[1]);
+        AssertThat(merged, IsFalse());
+        merged = ts_stack_push(stack, 1, stateA, trees[0]);
+        AssertThat(merged, IsTrue());
+
+        AssertThat(ts_stack_head_count(stack), Equals(1));
+        StackEntry *head = ts_stack_head(stack, 0);
+        AssertThat(*head, Equals<StackEntry>({trees[0], stateA}));
+
+        AssertThat(ts_stack_entry_next_count(head), Equals(2));
+        AssertThat(ts_stack_entry_next(head, 0), Equals<StackEntry *>(nullptr));
+        AssertThat(*ts_stack_entry_next(head, 1), Equals<StackEntry>({trees[1], stateB}));
+      });
+    });
   });
 
   describe("popping from a stack head that has been merged", [&]() {
diff --git a/src/runtime/stack.c b/src/runtime/stack.c
index bda889b1..3a4dd308 100644
--- a/src/runtime/stack.c
+++ b/src/runtime/stack.c
@@ -124,8 +124,11 @@ static void ts_stack__add_node_successor(Stack *self, StackNode *node,
                                          StackNode *new_successor) {
   for (int i = 0; i < node->successor_count; i++) {
     StackNode *successor = node->successors[i];
+    if (!successor)
+      continue;
     if (successor == new_successor)
       return;
+
     if (successor->entry.state == new_successor->entry.state) {
       if (successor->entry.tree != new_successor->entry.tree) {
         successor->entry.tree = self->tree_selection_callback.callback(