diff --git a/datasette/database.py b/datasette/database.py
index e162d34e..90c4e429 100644
--- a/datasette/database.py
+++ b/datasette/database.py
@@ -354,6 +354,15 @@ class Database:
                     result = fn(self._write_connection)
             else:
                 result = fn(self._write_connection)
+            if not block:
+                # There is no write thread here, so the write has already
+                # finished. Hand back the same (task_id, reply_future) shape
+                # _send_to_write_thread() returns, with the future already
+                # resolved, so the block=False path below is identical in
+                # both modes.
+                reply_future = asyncio.get_running_loop().create_future()
+                reply_future.set_result(result)
+                result = (uuid.uuid4(), reply_future)
         else:
             result = await self._send_to_write_thread(
                 fn, block=block, transaction=transaction
@@ -425,7 +434,7 @@ class Database:
             )
             self._write_thread.name = f"_execute_writes for database {self.name}"
             self._write_thread.start()
-        task_id = uuid.uuid5(uuid.NAMESPACE_DNS, "datasette.io")
+        task_id = uuid.uuid4()
         loop = asyncio.get_running_loop()
         reply_future = loop.create_future()
         self._write_queue.put(
diff --git a/tests/test_internals_database.py b/tests/test_internals_database.py
index b1093b1c..97513123 100644
--- a/tests/test_internals_database.py
+++ b/tests/test_internals_database.py
@@ -705,6 +705,33 @@ async def test_execute_write_fn_block_false(db):
     assert isinstance(task_id, uuid.UUID)
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("disable_threads", (False, True))
+async def test_execute_write_fn_block_false_returns_uuid(tmp_path, disable_threads):
+    # block=False is documented to return "a UUID representing the queued task".
+    # With num_sql_threads=0 there is no write thread, so the non-threaded branch
+    # has to satisfy the same contract as the threaded one.
+    settings = {"num_sql_threads": 0} if disable_threads else {}
+    ds = Datasette([], memory=True, settings=settings)
+    await ds.invoke_startup()
+    db = ds.add_memory_database("test_block_false")
+    await db.execute_write(
+        "create table if not exists t (id integer primary key, v text)"
+    )
+
+    def write_fn(conn):
+        conn.execute("insert into t (v) values ('a')")
+        # Returns None, like most write functions.
+
+    task_id = await db.execute_write_fn(write_fn, block=False)
+
+    assert isinstance(task_id, uuid.UUID)
+    # Distinct per call, so a caller can tell two queued tasks apart.
+    second = await db.execute_write_fn(write_fn, block=False)
+    assert isinstance(second, uuid.UUID)
+    assert second != task_id
+
+
 @pytest.mark.asyncio
 async def test_execute_write_fn_block_true(db):
     def write_fn(conn):