diff --git a/b2sdk/raw_api.py b/b2sdk/raw_api.py
index 5f2202f36..ee9cd6c98 100644
--- a/b2sdk/raw_api.py
+++ b/b2sdk/raw_api.py
@@ -758,6 +758,9 @@ def start_large_file(
             )
             kwargs['serverSideEncryption'] = server_side_encryption.serialize_to_json_for_request()
 
+            if server_side_encryption.mode == EncryptionMode.SSE_C:
+                file_info = server_side_encryption.add_key_id_to_file_info(file_info)
+
         if legal_hold is not None:
             kwargs['legalHold'] = legal_hold.to_server()
 
diff --git a/b2sdk/session.py b/b2sdk/session.py
index 09fc23e17..95bb790a0 100644
--- a/b2sdk/session.py
+++ b/b2sdk/session.py
@@ -508,9 +508,7 @@ def _add_app_key_info_to_unauthorized(self, unauthorized):
             key_messages.append('with no restrictions')
 
         # Make a new message
-        new_message = unauthorized.message
-        if new_message == '':
-            new_message = 'unauthorized'
+        new_message = unauthorized.message or 'unauthorized'
         new_message += ' for application key ' + ', '.join(key_messages)
 
         return Unauthorized(new_message, unauthorized.code)
diff --git a/test/integration/test_upload.py b/test/integration/test_upload.py
index 479047e49..d9ae6a5c3 100644
--- a/test/integration/test_upload.py
+++ b/test/integration/test_upload.py
@@ -11,8 +11,13 @@
 
 import io
 
+from b2sdk.b2http import B2Http
+from b2sdk.encryption.setting import EncryptionSetting, EncryptionKey
+from b2sdk.encryption.types import EncryptionMode, EncryptionAlgorithm
+from b2sdk.v2 import B2RawHTTPApi
 from .base import IntegrationTestBase
 from .fixtures import b2_auth_data  # noqa
+from .test_raw_api import authorize_raw_api
 
 
 class TestUnboundStreamUpload(IntegrationTestBase):
@@ -38,3 +43,38 @@ def test_streamed_large_buffer_small_part_size(self):
         data = b'a large data content' * 512 * 1024
         # 5mb, the smallest allowed part size
         self.assert_data_uploaded_via_stream(data, part_size=5 * 1024 * 1024)
+
+
+class TestUploadLargeFile(IntegrationTestBase):
+    def test_ssec_key_id(self):
+        sse_c = EncryptionSetting(
+            mode=EncryptionMode.SSE_C,
+            algorithm=EncryptionAlgorithm.AES256,
+            key=EncryptionKey(secret=b'********************************', key_id='some-id'),
+        )
+
+        raw_api = B2RawHTTPApi(B2Http())
+
+        auth_dict = authorize_raw_api(raw_api)
+        account_auth_token = auth_dict['authorizationToken']
+        api_url = auth_dict['apiUrl']
+        bucket = self.create_bucket()
+
+        large_info = raw_api.start_large_file(
+            api_url,
+            account_auth_token,
+            bucket.id_,
+            'test_largefile_sse_c.txt',
+            'text/plain',
+            None,
+            server_side_encryption=sse_c,
+        )
+
+        assert large_info['fileInfo'] == {
+            'sse_c_key_id': sse_c.key.key_id,
+        }
+        assert large_info['serverSideEncryption'] == {
+            'algorithm': 'AES256',
+            'customerKeyMd5': 'SaaDheEjzuynJH8eW6AEpQ==',
+            'mode': 'SSE-C',
+        }