diff --git a/sendgrid/helpers/mail/mail.py b/sendgrid/helpers/mail/mail.py
index 48d25f1..661c036 100644
--- a/sendgrid/helpers/mail/mail.py
+++ b/sendgrid/helpers/mail/mail.py
@@ -117,7 +117,11 @@ class Mail(object):
     def add_header(self, header):
         if self.headers is None:
             self.headers = []
-        self.headers.append(header)
+        if isinstance(header, dict):
+            (k,v) = list(header.items())[0]
+            self.headers.append(Header(k,v))
+        else:
+            self.headers.append(header)
 
     def add_category(self, category):
         if self.categories is None:

Using .add_header()  raises a TypeError
### Issue Summary
.add_header() cannot be used because python raises a TypeError exception.

### Steps to Reproduce
1. Create an instance of Mail `mail = Mail(  Email(from_email), subject, Email(to_emails), Content('text/html', message) )`
2. Add a header to the instance
`mail.add_header( {'Reply-To' : 'user@server.ext, user@server.ext'} )`
3. Send the Request
`response = sg.client.mail.send.post(request_body=mail.get())`

### Stack Trace
[File "mail.py", line 63](https://github.com/sendgrid/sendgrid-python/blob/master/sendgrid/helpers/mail/mail.py#L63), in **get**
> headers.update(key**.get()**)

TypeError: get expected at least 1 arguments, got 0
diff --git a/test/test_mail.py b/test/test_mail.py
index e0dc945..9b482dc 100644
--- a/test/test_mail.py
+++ b/test/test_mail.py
@@ -100,6 +100,8 @@ class UnitTests(unittest.TestCase):
         mail.add_header(Header("X-Test1", "test1"))
         mail.add_header(Header("X-Test3", "test2"))
 
+        mail.add_header({"X-Test4" : "test4"})
+
         mail.add_category(Category("May"))
         mail.add_category(Category("2016"))
 
@@ -131,4 +133,4 @@ class UnitTests(unittest.TestCase):
 
         mail.set_reply_to(Email("test@example.com"))
 
-        self.assertEqual(json.dumps(mail.get(), sort_keys=True), '{"asm": {"group_id": 99, "groups_to_display": [4, 5, 6, 7, 8]}, "attachments": [{"content": "TG9yZW0gaXBzdW0gZG9sb3Igc2l0IGFtZXQsIGNvbnNlY3RldHVyIGFkaXBpc2NpbmcgZWxpdC4gQ3JhcyBwdW12", "content_id": "Balance Sheet", "disposition": "attachment", "filename": "balance_001.pdf", "type": "application/pdf"}, {"content": "BwdW", "content_id": "Banner", "disposition": "inline", "filename": "banner.png", "type": "image/png"}], "batch_id": "sendgrid_batch_id", "categories": ["May", "2016"], "content": [{"type": "text/plain", "value": "some text here"}, {"type": "text/html", "value": "<html><body>some text here</body></html>"}], "custom_args": {"campaign": "welcome", "weekday": "morning"}, "from": {"email": "test@example.com", "name": "Example User"}, "headers": {"X-Test1": "test1", "X-Test3": "test2"}, "ip_pool_name": "24", "mail_settings": {"bcc": {"email": "test@example.com", "enable": true}, "bypass_list_management": {"enable": true}, "footer": {"enable": true, "html": "<html><body>Footer Text</body></html>", "text": "Footer Text"}, "sandbox_mode": {"enable": true}, "spam_check": {"enable": true, "post_to_url": "https://spamcatcher.sendgrid.com", "threshold": 1}}, "personalizations": [{"bcc": [{"email": "test@example.com"}, {"email": "test@example.com"}], "cc": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}], "custom_args": {"type": "marketing", "user_id": "343"}, "headers": {"X-Mock": "true", "X-Test": "test"}, "send_at": 1443636843, "subject": "Hello World from the Personalized SendGrid Python Library", "substitutions": {"%city%": "Denver", "%name%": "Example User"}, "to": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}]}, {"bcc": [{"email": "test@example.com"}, {"email": "test@example.com"}], "cc": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}], "custom_args": {"type": "marketing", "user_id": "343"}, "headers": {"X-Mock": "true", "X-Test": "test"}, "send_at": 1443636843, "subject": "Hello World from the Personalized SendGrid Python Library", "substitutions": {"%city%": "Denver", "%name%": "Example User"}, "to": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}]}], "reply_to": {"email": "test@example.com"}, "sections": {"%section1%": "Substitution Text for Section 1", "%section2%": "Substitution Text for Section 2"}, "send_at": 1443636842, "subject": "Hello World from the SendGrid Python Library", "template_id": "13b8f94f-bcae-4ec6-b752-70d6cb59f932", "tracking_settings": {"click_tracking": {"enable": true, "enable_text": true}, "ganalytics": {"enable": true, "utm_campaign": "some campaign", "utm_content": "some content", "utm_medium": "some medium", "utm_source": "some source", "utm_term": "some term"}, "open_tracking": {"enable": true, "substitution_tag": "Optional tag to replace with the open image in the body of the message"}, "subscription_tracking": {"enable": true, "html": "<html><body>html to insert into the text/html portion of the message</body></html>", "substitution_tag": "Optional tag to replace with the open image in the body of the message", "text": "text to insert into the text/plain portion of the message"}}}')
+        self.assertEqual(json.dumps(mail.get(), sort_keys=True), '{"asm": {"group_id": 99, "groups_to_display": [4, 5, 6, 7, 8]}, "attachments": [{"content": "TG9yZW0gaXBzdW0gZG9sb3Igc2l0IGFtZXQsIGNvbnNlY3RldHVyIGFkaXBpc2NpbmcgZWxpdC4gQ3JhcyBwdW12", "content_id": "Balance Sheet", "disposition": "attachment", "filename": "balance_001.pdf", "type": "application/pdf"}, {"content": "BwdW", "content_id": "Banner", "disposition": "inline", "filename": "banner.png", "type": "image/png"}], "batch_id": "sendgrid_batch_id", "categories": ["May", "2016"], "content": [{"type": "text/plain", "value": "some text here"}, {"type": "text/html", "value": "<html><body>some text here</body></html>"}], "custom_args": {"campaign": "welcome", "weekday": "morning"}, "from": {"email": "test@example.com", "name": "Example User"}, "headers": {"X-Test1": "test1", "X-Test3": "test2", "X-Test4": "test4"}, "ip_pool_name": "24", "mail_settings": {"bcc": {"email": "test@example.com", "enable": true}, "bypass_list_management": {"enable": true}, "footer": {"enable": true, "html": "<html><body>Footer Text</body></html>", "text": "Footer Text"}, "sandbox_mode": {"enable": true}, "spam_check": {"enable": true, "post_to_url": "https://spamcatcher.sendgrid.com", "threshold": 1}}, "personalizations": [{"bcc": [{"email": "test@example.com"}, {"email": "test@example.com"}], "cc": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}], "custom_args": {"type": "marketing", "user_id": "343"}, "headers": {"X-Mock": "true", "X-Test": "test"}, "send_at": 1443636843, "subject": "Hello World from the Personalized SendGrid Python Library", "substitutions": {"%city%": "Denver", "%name%": "Example User"}, "to": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}]}, {"bcc": [{"email": "test@example.com"}, {"email": "test@example.com"}], "cc": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}], "custom_args": {"type": "marketing", "user_id": "343"}, "headers": {"X-Mock": "true", "X-Test": "test"}, "send_at": 1443636843, "subject": "Hello World from the Personalized SendGrid Python Library", "substitutions": {"%city%": "Denver", "%name%": "Example User"}, "to": [{"email": "test@example.com", "name": "Example User"}, {"email": "test@example.com", "name": "Example User"}]}], "reply_to": {"email": "test@example.com"}, "sections": {"%section1%": "Substitution Text for Section 1", "%section2%": "Substitution Text for Section 2"}, "send_at": 1443636842, "subject": "Hello World from the SendGrid Python Library", "template_id": "13b8f94f-bcae-4ec6-b752-70d6cb59f932", "tracking_settings": {"click_tracking": {"enable": true, "enable_text": true}, "ganalytics": {"enable": true, "utm_campaign": "some campaign", "utm_content": "some content", "utm_medium": "some medium", "utm_source": "some source", "utm_term": "some term"}, "open_tracking": {"enable": true, "substitution_tag": "Optional tag to replace with the open image in the body of the message"}, "subscription_tracking": {"enable": true, "html": "<html><body>html to insert into the text/html portion of the message</body></html>", "substitution_tag": "Optional tag to replace with the open image in the body of the message", "text": "text to insert into the text/plain portion of the message"}}}')

pip install -e .
parse_log_pytest
requirements.txt
unittest2 unittest2py3k pytest
3.5
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs==22.2.0
certifi==2021.5.30
dataclasses==0.8
Flask==0.10.1
importlib-metadata==4.8.3
iniconfig==1.1.1
itsdangerous==2.0.1
Jinja2==3.0.3
linecache2==1.0.0
MarkupSafe==2.0.1
packaging==21.3
pluggy==1.0.0
py==1.11.0
pyparsing==3.1.4
pytest==7.0.1
python-http-client==2.2.1
PyYAML==3.11
-e git+https://github.com/sendgrid/sendgrid-python.git@ca96c8dcd66224e13b38ab8fd2d2b429dd07dd02#egg=sendgrid
six==1.17.0
tomli==1.2.3
traceback2==1.4.0
typing_extensions==4.1.1
unittest2==1.1.0
unittest2py3k==0.5.1
Werkzeug==2.0.3
zipp==3.6.0

name: sendgrid-python
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=2021.5.30=py36h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - 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=21.2.2=py36h06a4308_0
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - argparse==1.4.0
      - attrs==22.2.0
      - dataclasses==0.8
      - flask==0.10.1
      - importlib-metadata==4.8.3
      - iniconfig==1.1.1
      - itsdangerous==2.0.1
      - jinja2==3.0.3
      - linecache2==1.0.0
      - markupsafe==2.0.1
      - packaging==21.3
      - pluggy==1.0.0
      - py==1.11.0
      - pyparsing==3.1.4
      - pytest==7.0.1
      - python-http-client==2.2.1
      - pyyaml==3.11
      - six==1.17.0
      - tomli==1.2.3
      - traceback2==1.4.0
      - typing-extensions==4.1.1
      - unittest2==1.1.0
      - unittest2py3k==0.5.1
      - werkzeug==2.0.3
      - zipp==3.6.0
prefix: /opt/conda/envs/sendgrid-python

test/test_mail.py::UnitTests::test_kitchenSink
test/test_mail.py::UnitTests::test_helloEmail
sendgrid/helpers/mail/mail.py
sendgrid/helpers/mail/mail.py