diff --git a/rag/prompts/generator.py b/rag/prompts/generator.py
index 2ef8b8f8c8..ddf99251b5 100644
--- a/rag/prompts/generator.py
+++ b/rag/prompts/generator.py
@@ -129,7 +129,7 @@ def kb_prompt(kbinfos, max_tokens, hash_id=False):
         cnt = "\nID: {}".format(i if not hash_id else hash_str2int(get_value(ck, "id", "chunk_id"), 500))
         cnt += draw_node("Title", get_value(ck, "docnm_kwd", "document_name"))
         cnt += draw_node("URL", ck.get('url', ''))
-        meta = ck.get("document_metadata", {})
+        meta = ck.get("document_metadata") or {}
         for k, v in meta.items():
             cnt += draw_node(k, v)
         cnt += "\n└── Content:\n"
diff --git a/test/unit_test/rag/prompts/test_kb_prompt_metadata.py b/test/unit_test/rag/prompts/test_kb_prompt_metadata.py
new file mode 100644
index 0000000000..86d96eeec3
--- /dev/null
+++ b/test/unit_test/rag/prompts/test_kb_prompt_metadata.py
@@ -0,0 +1,87 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+import pytest
+
+from rag.prompts.generator import kb_prompt
+
+
+@pytest.mark.p1
+class TestKbPromptDocumentMetadata:
+    """Regression tests for kb_prompt's handling of `document_metadata` on chunks."""
+
+    @pytest.mark.p1
+    def test_null_document_metadata_does_not_crash(self):
+        """A chunk with `document_metadata: None` must not raise AttributeError.
+
+        Regression for issue #14651: chunks retrieved from the index can carry
+        an explicit null metadata field, which made `dict.get(..., {})` return
+        `None` and crash citation generation with
+        `AttributeError: 'NoneType' object has no attribute 'items'`.
+        """
+        kbinfos = {
+            "chunks": [
+                {
+                    "id": "chunk-1",
+                    "content_with_weight": "hello world",
+                    "docnm_kwd": "doc.pdf",
+                    "document_metadata": None,
+                }
+            ]
+        }
+
+        rendered = kb_prompt(kbinfos, max_tokens=10000)
+
+        assert len(rendered) == 1
+        assert "hello world" in rendered[0]
+        assert "doc.pdf" in rendered[0]
+
+    @pytest.mark.p1
+    def test_missing_document_metadata_key(self):
+        """A chunk with no `document_metadata` key at all should also work."""
+        kbinfos = {
+            "chunks": [
+                {
+                    "id": "chunk-1",
+                    "content_with_weight": "hello world",
+                    "docnm_kwd": "doc.pdf",
+                }
+            ]
+        }
+
+        rendered = kb_prompt(kbinfos, max_tokens=10000)
+
+        assert len(rendered) == 1
+        assert "hello world" in rendered[0]
+
+    @pytest.mark.p1
+    def test_populated_document_metadata_renders_fields(self):
+        """When metadata is a dict, its key/value pairs must be rendered."""
+        kbinfos = {
+            "chunks": [
+                {
+                    "id": "chunk-1",
+                    "content_with_weight": "hello world",
+                    "docnm_kwd": "doc.pdf",
+                    "document_metadata": {"author": "alice", "year": "2026"},
+                }
+            ]
+        }
+
+        rendered = kb_prompt(kbinfos, max_tokens=10000)
+
+        assert len(rendered) == 1
+        assert "author: alice" in rendered[0]
+        assert "year: 2026" in rendered[0]