diff --git a/src/keycloak/keycloak_openid.py b/src/keycloak/keycloak_openid.py
index 94144de..5e556e9 100644
--- a/src/keycloak/keycloak_openid.py
+++ b/src/keycloak/keycloak_openid.py
@@ -1424,6 +1424,8 @@ class KeycloakOpenID:
         params_path = {"realm-name": self.realm_name}
         orig_bearer = self.connection.headers.get("Authorization")
         self.connection.add_param_headers("Authorization", "Bearer " + token)
+        orig_content_type = self.connection.headers.get("Content-Type")
+        self.connection.add_param_headers("Content-Type", "application/json")
         data_raw = await self.connection.a_raw_post(
             URL_CLIENT_REGISTRATION.format(**params_path), data=json.dumps(payload)
         )
@@ -1432,6 +1434,11 @@ class KeycloakOpenID:
             if orig_bearer is not None
             else self.connection.del_param_headers("Authorization")
         )
+        (
+            self.connection.add_param_headers("Content-Type", orig_content_type)
+            if orig_content_type is not None
+            else self.connection.del_param_headers("Content-Type")
+        )
         return raise_error_from_response(data_raw, KeycloakPostError)
 
     async def a_device(self):
@@ -1477,6 +1484,8 @@ class KeycloakOpenID:
         params_path = {"realm-name": self.realm_name, "client-id": client_id}
         orig_bearer = self.connection.headers.get("Authorization")
         self.connection.add_param_headers("Authorization", "Bearer " + token)
+        orig_content_type = self.connection.headers.get("Content-Type")
+        self.connection.add_param_headers("Content-Type", "application/json")
 
         # Keycloak complains if the clientId is not set in the payload
         if "clientId" not in payload:
@@ -1490,4 +1499,9 @@ class KeycloakOpenID:
             if orig_bearer is not None
             else self.connection.del_param_headers("Authorization")
         )
+        (
+            self.connection.add_param_headers("Content-Type", orig_content_type)
+            if orig_content_type is not None
+            else self.connection.del_param_headers("Content-Type")
+        )
         return raise_error_from_response(data_raw, KeycloakPutError)
diff --git a/tests/test_keycloak_admin.py b/tests/test_keycloak_admin.py
index d57654f..8a61faa 100644
--- a/tests/test_keycloak_admin.py
+++ b/tests/test_keycloak_admin.py
@@ -6114,7 +6114,7 @@ async def test_a_initial_access_token(
     client = str(uuid.uuid4())
     secret = str(uuid.uuid4())
 
-    res = oid.register_client(
+    res = await oid.a_register_client(
         token=res["token"],
         payload={
             "name": "DynamicRegisteredClient",