diff --git a/src/githubapp/core.py b/src/githubapp/core.py
index 34517e3..944a2ab 100644
--- a/src/githubapp/core.py
+++ b/src/githubapp/core.py
@@ -576,8 +576,14 @@ def _create_jwt(self, expiration=60):
         :param expiration: int
         :return string:
         """
+        if self.id is None:
+            raise GitHubAppError(
+                message="GitHub App ID (github_app_id) is not configured; cannot generate JWT.",
+                status=None,
+                data=None,
+            )
         now = int(time.time())
-        payload = {"iat": now, "exp": now + expiration, "iss": self.id}
+        payload = {"iat": now, "exp": now + expiration, "iss": str(self.id)}
         encrypted = jwt.encode(payload, key=self.key, algorithm="RS256")
 
         if isinstance(encrypted, bytes):
diff --git a/tests/test_core.py b/tests/test_core.py
index 5b53e24..adb048d 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -202,7 +202,7 @@ def test_create_jwt_payload_structure(self, mock_time, mock_jwt):
         github_app = GitHubApp(github_app_id=123, github_app_key=b"test_key")
         github_app._create_jwt(expiration=300)
 
-        expected_payload = {"iat": 1640995200, "exp": 1640995200 + 300, "iss": 123}
+        expected_payload = {"iat": 1640995200, "exp": 1640995200 + 300, "iss": "123"}
         mock_jwt.encode.assert_called_once_with(
             expected_payload, key=b"test_key", algorithm="RS256"
         )