diff --git a/rerankers/utils.py b/rerankers/utils.py
index cf6aaaf..d7d4e99 100644
--- a/rerankers/utils.py
+++ b/rerankers/utils.py
@@ -93,6 +93,11 @@ def prep_docs(
                     "'None' doc_ids detected, reverting to auto-generated integer ids..."
                 )
                 doc_ids = list(range(len(docs)))
+                # Write the generated ids back onto the documents (the string-input
+                # path below already does this); otherwise every returned Document
+                # keeps doc_id=None and get_score_by_docid/get_result_by_docid break.
+                for doc, doc_id in zip(docs, doc_ids):
+                    doc.doc_id = doc_id
 
         if metadata is not None:
             if docs[0].metadata is not None:
diff --git a/tests/test_results.py b/tests/test_results.py
index e0d7deb..be02f55 100644
--- a/tests/test_results.py
+++ b/tests/test_results.py
@@ -31,3 +31,13 @@ def test_result_validation_error():
     with pytest.raises(ValueError) as excinfo:
         Result(document=Document(doc_id=2, text="Doc 2"))
     assert "Either score or rank must be provided." in str(excinfo.value)
+
+
+def test_prep_docs_document_input_autogenerates_ids():
+    from rerankers.utils import prep_docs
+
+    # Document objects with no explicit doc_id must receive auto-generated
+    # integer ids so get_score_by_docid / get_result_by_docid can resolve them.
+    docs = [Document(text="a"), Document(text="b"), Document(text="c")]
+    out = prep_docs(docs)
+    assert [d.doc_id for d in out] == [0, 1, 2]