diff --git a/slackeventsapi/server.py b/slackeventsapi/server.py
index c1436e5..74afca5 100644
--- a/slackeventsapi/server.py
+++ b/slackeventsapi/server.py
@@ -89,7 +89,7 @@ class SlackServer(Flask):
             # Each request comes with request timestamp and request signature
             # emit an error if the timestamp is out of range
             req_timestamp = request.headers.get('X-Slack-Request-Timestamp')
-            if abs(time() - int(req_timestamp)) > 60 * 5:
+            if req_timestamp is None or abs(time() - int(req_timestamp)) > 60 * 5:
                 slack_exception = SlackEventAdapterException('Invalid request timestamp')
                 self.emitter.emit('error', slack_exception)
                 return make_response("", 403)
@@ -97,7 +97,7 @@ class SlackServer(Flask):
             # Verify the request signature using the app's signing secret
             # emit an error if the signature can't be verified
             req_signature = request.headers.get('X-Slack-Signature')
-            if not self.verify_signature(req_timestamp, req_signature):
+            if req_signature is None or not self.verify_signature(req_timestamp, req_signature):
                 slack_exception = SlackEventAdapterException('Invalid request signature')
                 self.emitter.emit('error', slack_exception)
                 return make_response("", 403)

Lack of X-Slack-Request-Timestamp and/or X-Slack-Signature in requests results in 500
### Description

I noticed that a test I wrote to simulate a user probing the events URL returned a 500 when I would have expected a 400.

### What type of issue is this? (place an `x` in one of the `[ ]`)
- [x] bug
- [ ] enhancement (feature request)
- [ ] question
- [ ] documentation related
- [ ] testing related
- [ ] discussion

### Requirements
* [x] I've read and understood the [Contributing guidelines](https://github.com/slackapi/python-slack-events-api/blob/master/.github/contributing.md) and have done my best effort to follow them. -- _However, Spotify, my employer, has not yet signed the CLA to my knowledge, so I will not be able to submit a patch until they do._
* [x] I've read and agree to the [Code of Conduct](https://slackhq.github.io/code-of-conduct).
* [x] I've searched for any related issues and avoided creating a duplicate issue.

### Bug Report

Filling out the following details about bugs will help us solve your issue sooner.

#### Reproducible in:
slackeventsapi version: 2.2.0
python version: 3.6
OS version(s): irrelevant, but Linux and MacOS

#### Steps to reproduce:

1. Make a POST request to a slack events URL without `X-Slack-Events-Timestamp` in the headers.
2. Observe response of 500 and subsequent TypeError raised

#### Expected result:

A 400 error.

#### Actual result:

500 and unhandled exception.

#### Attachments:

diff --git a/tests/test_server.py b/tests/test_server.py
index 172d475..a66a560 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -56,6 +56,32 @@ def test_url_challenge(client):
     assert bytes.decode(res.data) == "valid_challenge_token"
 
 
+def test_no_request_timestamp_header(client):
+    data = pytest.reaction_event_fixture
+    with pytest.raises(SlackEventAdapterException) as excinfo:
+        res = client.post(
+            '/slack/events',
+            data=data,
+            content_type='application/json',
+            headers={}
+        )
+    assert str(excinfo.value) == 'Invalid request timestamp'
+
+def test_no_request_signature_header(client):
+    data = pytest.reaction_event_fixture
+    timestamp = int(time.time())
+    with pytest.raises(SlackEventAdapterException) as excinfo:
+        res = client.post(
+            '/slack/events',
+            data=data,
+            content_type='application/json',
+            headers={
+                'X-Slack-Request-Timestamp': timestamp, # valid
+            }
+        )
+    assert str(excinfo.value) == 'Invalid request signature'
+
+
 def test_invalid_request_signature(client):
     # Verify [package metadata header is set
     slack_adapter = SlackEventAdapter("SIGNING_SECRET")

pip install -e .
parse_log_pytest
requirements.txt
pytest pytest-cov pytest-mock pytest-flask
3.7
example/requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
aiohttp==3.8.6
aiosignal==1.3.1
async-timeout==4.0.3
asynctest==0.13.0
attrs==24.2.0
certifi @ file:///croot/certifi_1671487769961/work/certifi
charset-normalizer==3.4.1
click==7.1.2
coverage==7.2.7
exceptiongroup==1.2.2
Flask==1.1.4
frozenlist==1.3.3
idna==3.10
importlib-metadata==6.7.0
iniconfig==2.0.0
itsdangerous==1.1.0
Jinja2==2.11.3
MarkupSafe==1.1.1
multidict==6.0.5
packaging==24.0
pluggy==1.2.0
pyee==7.0.4
pytest==7.4.4
pytest-cov==4.1.0
pytest-flask==1.3.0
pytest-mock==3.11.1
slackclient==2.9.4
-e git+https://github.com/slackapi/python-slack-events-api.git@2d298a9b90b5b4a5599d120bd0acb45a1b77fa4b#egg=slackeventsapi
tomli==2.0.1
typing_extensions==4.7.1
Werkzeug==1.0.1
yarl==1.9.4
zipp==3.15.0

name: python-slack-events-api
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2022.12.7=py37h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=22.3.1=py37h06a4308_0
  - python=3.7.16=h7a1cb2a_0
  - readline=8.2=h5eee18b_0
  - setuptools=65.6.3=py37h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.38.4=py37h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - aiohttp==3.8.6
      - aiosignal==1.3.1
      - async-timeout==4.0.3
      - asynctest==0.13.0
      - attrs==24.2.0
      - charset-normalizer==3.4.1
      - click==7.1.2
      - coverage==7.2.7
      - exceptiongroup==1.2.2
      - flask==1.1.4
      - frozenlist==1.3.3
      - idna==3.10
      - importlib-metadata==6.7.0
      - iniconfig==2.0.0
      - itsdangerous==1.1.0
      - jinja2==2.11.3
      - markupsafe==1.1.1
      - multidict==6.0.5
      - packaging==24.0
      - pluggy==1.2.0
      - pyee==7.0.4
      - pytest==7.4.4
      - pytest-cov==4.1.0
      - pytest-flask==1.3.0
      - pytest-mock==3.11.1
      - slackclient==2.9.4
      - tomli==2.0.1
      - typing-extensions==4.7.1
      - werkzeug==1.0.1
      - yarl==1.9.4
      - zipp==3.15.0
prefix: /opt/conda/envs/python-slack-events-api

tests/test_server.py::test_no_request_timestamp_header tests/test_server.py::test_no_request_signature_header
tests/test_server.py::test_existing_flask tests/test_server.py::test_server_not_flask tests/test_server.py::test_blueprint_server tests/test_server.py::test_event_endpoint_get tests/test_server.py::test_url_challenge tests/test_server.py::test_invalid_request_signature tests/test_server.py::test_invalid_request_timestamp tests/test_server.py::test_compare_digest_fallback tests/test_server.py::test_version_header tests/test_server.py::test_server_start tests/test_server.py::test_default_exception_msg
slackeventsapi/server.py
slackeventsapi/server.py