diff --git a/monzo/httpio.py b/monzo/httpio.py
index 1fd6b11..6ccb039 100644
--- a/monzo/httpio.py
+++ b/monzo/httpio.py
@@ -195,7 +195,8 @@ def _perform_request(
             with response as fh:
                 content += fh.read().decode('utf-8')
         except HTTPError as error:
-            raise MONZO_ERROR_MAP[error.code]() from error
+            exception_cls = MONZO_ERROR_MAP.get(error.code, MonzoGeneralError)
+            raise exception_cls() from error
         return {
             'code': response.code,
             'headers': response.headers,
diff --git a/tests/test_httpio_errors.py b/tests/test_httpio_errors.py
new file mode 100644
index 0000000..3292221
--- /dev/null
+++ b/tests/test_httpio_errors.py
@@ -0,0 +1,15 @@
+from unittest.mock import patch
+from urllib.error import HTTPError
+
+import pytest
+
+from monzo.exceptions import MonzoGeneralError
+from monzo.httpio import HttpIO
+
+
+def test_unknown_status_code_raises_monzogeneralerror():
+    http = HttpIO('https://example.com')
+    error = HTTPError(url='https://example.com/test', code=418, msg='teapot', hdrs=None, fp=None)
+    with patch('monzo.httpio.urlopen', side_effect=error):
+        with pytest.raises(MonzoGeneralError):
+            http.get('/test')