diff --git a/CHANGELOG.asciidoc b/CHANGELOG.asciidoc
index cf4dbeae9..eef92bc4d 100644
--- a/CHANGELOG.asciidoc
+++ b/CHANGELOG.asciidoc
@@ -48,6 +48,7 @@ endif::[]
 * Restrict length of Starlette request bodies {pull}1549[#1549]
 * Fix error when using elasticsearch(sniff_on_start=True) {pull}1618[#1618]
 * Improve handling of ignored URLs and capture_body=off for Starlette {pull}1549[#1549]
+* Fix possible error in the transport flush for Lambda functions {pull}1628[#1628]
 
 
 
diff --git a/elasticapm/transport/base.py b/elasticapm/transport/base.py
index d0abb2a37..f0485c4b2 100644
--- a/elasticapm/transport/base.py
+++ b/elasticapm/transport/base.py
@@ -176,11 +176,11 @@ def _process_queue(self):
             if flush:
                 if buffer_written:
                     self._flush(buffer, forced_flush=forced_flush)
-                elif forced_flush and "/localhost:" in self.client.config.server_url:
+                elif forced_flush and any(x in self.client.config.server_url for x in ("/localhost:", "/127.0.0.1:")):
                     # No data on buffer, but due to manual flush we should send
                     # an empty payload with flushed=true query param, but only
                     # to a local APM server (or lambda extension)
-                    self.send(None, flushed=True)
+                    self.send("", forced_flush=True)
                 self._last_flush = timeit.default_timer()
                 buffer = self._init_buffer()
                 buffer_written = False
diff --git a/tests/transports/test_base.py b/tests/transports/test_base.py
index 6baa52c0c..e4c0e88fc 100644
--- a/tests/transports/test_base.py
+++ b/tests/transports/test_base.py
@@ -264,3 +264,13 @@ def test_flushed_arg(sending_elasticapm_client):
     sending_elasticapm_client._transport.flush()
 
     assert sending_elasticapm_client.httpserver.requests[0].args["flushed"] == "true"
+
+
+@pytest.mark.parametrize("sending_elasticapm_client", [{"api_request_time": "100ms"}], indirect=True)
+def test_flushed_arg_with_wait(sending_elasticapm_client):
+    sending_elasticapm_client.begin_transaction("test_type")
+    sending_elasticapm_client.end_transaction("test")
+    time.sleep(0.2)
+    sending_elasticapm_client._transport.flush()
+
+    assert sending_elasticapm_client.httpserver.requests[1].args["flushed"] == "true"