diff --git a/spacy/tests/doc/test_json_doc_conversion.py b/spacy/tests/doc/test_json_doc_conversion.py
index 11a1817e6..26dd3af0f 100644
--- a/spacy/tests/doc/test_json_doc_conversion.py
+++ b/spacy/tests/doc/test_json_doc_conversion.py
@@ -313,7 +313,7 @@ def test_json_to_doc_spans(doc):
     """Test that Doc.from_json() includes correct.spans."""
     doc.spans["test"] = [
         Span(doc, 0, 2, label="test"),
-        Span(doc, 0, 1, label="test", kb_id=7),
+        Span(doc, 0, 1, label="test", kb_id="7"),
     ]
     json_doc = doc.to_json()
     new_doc = Doc(doc.vocab).from_json(json_doc, validate=True)
diff --git a/spacy/tests/parser/test_ner.py b/spacy/tests/parser/test_ner.py
index 030182a63..a2cf8f079 100644
--- a/spacy/tests/parser/test_ner.py
+++ b/spacy/tests/parser/test_ner.py
@@ -599,22 +599,22 @@ def test_overfitting_IO(use_upper):
     # test that kb_id is preserved
     test_text = "I like London and London."
     doc = nlp.make_doc(test_text)
-    doc.ents = [Span(doc, 2, 3, label="LOC", kb_id=1234)]
+    doc.ents = [Span(doc, 2, 3, label="LOC", kb_id="1234")]
     ents = doc.ents
     assert len(ents) == 1
     assert ents[0].text == "London"
     assert ents[0].label_ == "LOC"
-    assert ents[0].kb_id == 1234
+    assert ents[0].kb_id_ == "1234"
     doc = nlp.get_pipe("ner")(doc)
     ents = doc.ents
     assert len(ents) == 2
     assert ents[0].text == "London"
     assert ents[0].label_ == "LOC"
-    assert ents[0].kb_id == 1234
-    # ent added by ner has kb_id == 0
+    assert ents[0].kb_id_ == "1234"
+    # ent added by ner has kb_id == ""
     assert ents[1].text == "London"
     assert ents[1].label_ == "LOC"
-    assert ents[1].kb_id == 0
+    assert ents[1].kb_id_ == ""
 
 
 def test_beam_ner_scores():
diff --git a/spacy/tokens/_serialize.py b/spacy/tokens/_serialize.py
index c4e8f26f4..015bf088d 100644
--- a/spacy/tokens/_serialize.py
+++ b/spacy/tokens/_serialize.py
@@ -124,6 +124,8 @@ class DocBin:
         for key, group in doc.spans.items():
             for span in group:
                 self.strings.add(span.label_)
+                self.strings.add(span.kb_id_)
+                self.strings.add(span.id_)
 
     def get_docs(self, vocab: Vocab) -> Iterator[Doc]:
         """Recover Doc objects from the annotations, using the given vocab.
diff --git a/spacy/tokens/doc.pyx b/spacy/tokens/doc.pyx
index 7dfe0ca9f..4ff946bde 100644
--- a/spacy/tokens/doc.pyx
+++ b/spacy/tokens/doc.pyx
@@ -1350,6 +1350,8 @@ cdef class Doc:
         for group in self.spans.values():
             for span in group:
                 strings.add(span.label_)
+                strings.add(span.kb_id_)
+                strings.add(span.id_)
         # Msgpack doesn't distinguish between lists and tuples, which is
         # vexing for user data. As a best guess, we *know* that within
         # keys, we must have tuples. In values we just have to hope