diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 3c7ed4a..a85d32e 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -52,6 +52,10 @@ Fixes
 * **Postmark:** Handle Postmark's SubscriptionChange events as Anymail
   unsubscribe, subscribe, or bounce tracking events, rather than "unknown".
   (Thanks to `@puru02`_ for the fix.)
+* **Sendinblue:** Work around recent (unannounced) Sendinblue API change
+  that caused "Invalid headers" API error with non-string custom header
+  values. Anymail now converts int and float header values to strings.
+
 
 Other
 ~~~~~
diff --git a/anymail/backends/sendinblue.py b/anymail/backends/sendinblue.py
index 4bfff08..4db75af 100644
--- a/anymail/backends/sendinblue.py
+++ b/anymail/backends/sendinblue.py
@@ -3,7 +3,7 @@ from requests.structures import CaseInsensitiveDict
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 from ..exceptions import AnymailRequestsAPIError
 from ..message import AnymailRecipientStatus
-from ..utils import get_anymail_setting
+from ..utils import get_anymail_setting, BASIC_NUMERIC_TYPES
 
 
 class EmailBackend(AnymailRequestsBackend):
@@ -112,7 +112,12 @@ class SendinBluePayload(RequestsPayload):
             self.data['replyTo'] = self.email_object(emails[0])
 
     def set_extra_headers(self, headers):
-        self.data['headers'].update(headers)
+        # SendinBlue requires header values to be strings -- not integers -- as of 11/2022.
+        # We'll stringify ints and floats; anything else is the caller's responsibility.
+        self.data["headers"].update({
+            k: str(v) if isinstance(v, BASIC_NUMERIC_TYPES) else v
+            for k, v in headers.items()
+        })
 
     def set_tags(self, tags):
         if len(tags) > 0:
diff --git a/tests/test_sendinblue_backend.py b/tests/test_sendinblue_backend.py
index b83be90..f053f42 100644
--- a/tests/test_sendinblue_backend.py
+++ b/tests/test_sendinblue_backend.py
@@ -118,7 +118,8 @@ class SendinBlueBackendStandardEmailTests(SendinBlueBackendMockAPITestCase):
         self.message.send()
         data = self.get_api_call_json()
         self.assertEqual(data['headers']['X-Custom'], 'string')
-        self.assertEqual(data['headers']['X-Num'], 123)
+        # Header values must be strings (changed 11/2022)
+        self.assertEqual(data['headers']['X-Num'], "123")
         # Reply-To must be moved to separate param
         self.assertNotIn('Reply-To', data['headers'])
         self.assertEqual(data['replyTo'], {'name': "Do Not Reply", 'email': "noreply@example.com"})