diff --git a/src/runtime/tree.c b/src/runtime/tree.c
index 92003ff1..336aaab4 100644
--- a/src/runtime/tree.c
+++ b/src/runtime/tree.c
@@ -553,13 +553,19 @@ void ts_tree_print_dot_graph(const Tree *self, const TSLanguage *language,
   fprintf(f, "}\n");
 }
 
+TSExternalTokenState empty_state = {
+  0, 0, 0, 0,
+  0, 0, 0, 0,
+  0, 0, 0, 0,
+  0, 0, 0, 0,
+};
+
 bool ts_tree_external_token_state_eq(const Tree *self, const Tree *other) {
-  return self == other || (
-    self &&
-    other &&
-    self->has_external_tokens == other->has_external_tokens && (
-      !self->has_external_tokens ||
-      memcmp(&self->external_token_state, &other->external_token_state, sizeof(TSExternalTokenState)) == 0
-    )
-  );
+  const TSExternalTokenState *state1 = &empty_state;
+  const TSExternalTokenState *state2 = &empty_state;
+  if (self && self->has_external_tokens) state1 = &self->external_token_state;
+  if (other && other->has_external_tokens) state2 = &other->external_token_state;
+  return
+    state1 == state2 ||
+    memcmp(state1, state2, sizeof(TSExternalTokenState)) == 0;
 }
diff --git a/test/runtime/stack_test.cc b/test/runtime/stack_test.cc
index 14bcbc7c..ecf579e0 100644
--- a/test/runtime/stack_test.cc
+++ b/test/runtime/stack_test.cc
@@ -524,8 +524,10 @@ describe("Stack", [&]() {
 
   describe("setting external token state", [&]() {
     before_each([&]() {
-      trees[1]->external_token_state[0] = 'a';
-      trees[2]->external_token_state[0] = 'b';
+      trees[1]->has_external_tokens = true;
+      trees[2]->has_external_tokens = true;
+      memset(&trees[1]->external_token_state, 0, sizeof(TSExternalTokenState));
+      memset(&trees[2]->external_token_state, 0, sizeof(TSExternalTokenState));
     });
 
     it("allows the state to be retrieved", [&]() {
@@ -535,19 +537,49 @@ describe("Stack", [&]() {
       AssertThat(ts_stack_last_external_token(stack, 0), Equals(trees[1]));
 
       ts_stack_copy_version(stack, 0);
-      AssertThat(ts_stack_last_external_token(stack, 0), Equals(trees[1]));
+      AssertThat(ts_stack_last_external_token(stack, 1), Equals(trees[1]));
+
+      ts_stack_set_last_external_token(stack, 0, trees[2]);
+      AssertThat(ts_stack_last_external_token(stack, 0), Equals(trees[2]));
     });
 
     it("does not merge stack versions with different external token states", [&]() {
+      trees[1]->external_token_state[5] = 'a';
+      trees[2]->external_token_state[5] = 'b';
+
       ts_stack_copy_version(stack, 0);
       ts_stack_push(stack, 0, trees[0], false, 5);
       ts_stack_push(stack, 1, trees[0], false, 5);
 
       ts_stack_set_last_external_token(stack, 0, trees[1]);
-      ts_stack_set_last_external_token(stack, 0, trees[1]);
+      ts_stack_set_last_external_token(stack, 1, trees[2]);
 
       AssertThat(ts_stack_merge(stack, 0, 1), IsFalse());
     });
+
+    it("merges stack versions with identical external token states", [&]() {
+      trees[1]->external_token_state[5] = 'a';
+      trees[2]->external_token_state[5] = 'a';
+
+      ts_stack_copy_version(stack, 0);
+      ts_stack_push(stack, 0, trees[0], false, 5);
+      ts_stack_push(stack, 1, trees[0], false, 5);
+
+      ts_stack_set_last_external_token(stack, 0, trees[1]);
+      ts_stack_set_last_external_token(stack, 1, trees[2]);
+
+      AssertThat(ts_stack_merge(stack, 0, 1), IsTrue());
+    });
+
+    it("does not distinguish between an *empty* external token state and *no* external token state", [&]() {
+      ts_stack_copy_version(stack, 0);
+      ts_stack_push(stack, 0, trees[0], false, 5);
+      ts_stack_push(stack, 1, trees[0], false, 5);
+
+      ts_stack_set_last_external_token(stack, 0, trees[1]);
+
+      AssertThat(ts_stack_merge(stack, 0, 1), IsTrue());
+    });
   });
 });