diff --git a/spacy/tests/serialize/test_serialize_doc.py b/spacy/tests/serialize/test_serialize_doc.py
index 15bf67bfd..eea13445e 100644
--- a/spacy/tests/serialize/test_serialize_doc.py
+++ b/spacy/tests/serialize/test_serialize_doc.py
@@ -213,6 +213,13 @@ def test_serialize_doc_exclude(en_vocab):
 
 def test_serialize_doc_span_groups(en_vocab):
     doc = Doc(en_vocab, words=["hello", "world", "!"])
-    doc.spans["content"] = [doc[0:2]]
+    span = doc[0:2]
+    span.label_ = "test_serialize_doc_span_groups_label"
+    span.id_ = "test_serialize_doc_span_groups_id"
+    span.kb_id_ = "test_serialize_doc_span_groups_kb_id"
+    doc.spans["content"] = [span]
     new_doc = Doc(en_vocab).from_bytes(doc.to_bytes())
     assert len(new_doc.spans["content"]) == 1
+    assert new_doc.spans["content"][0].label_ == "test_serialize_doc_span_groups_label"
+    assert new_doc.spans["content"][0].id_ == "test_serialize_doc_span_groups_id"
+    assert new_doc.spans["content"][0].kb_id_ == "test_serialize_doc_span_groups_kb_id"
diff --git a/spacy/tests/serialize/test_serialize_docbin.py b/spacy/tests/serialize/test_serialize_docbin.py
index 9f8e5e06b..6f7b1001c 100644
--- a/spacy/tests/serialize/test_serialize_docbin.py
+++ b/spacy/tests/serialize/test_serialize_docbin.py
@@ -49,7 +49,11 @@ def test_serialize_doc_bin():
     nlp = English()
     for doc in nlp.pipe(texts):
         doc.cats = cats
-        doc.spans["start"] = [doc[0:2]]
+        span = doc[0:2]
+        span.label_ = "UNUSUAL_SPAN_LABEL"
+        span.id_ = "UNUSUAL_SPAN_ID"
+        span.kb_id_ = "UNUSUAL_SPAN_KB_ID"
+        doc.spans["start"] = [span]
         doc[0].norm_ = "UNUSUAL_TOKEN_NORM"
         doc[0].ent_id_ = "UNUSUAL_TOKEN_ENT_ID"
         doc_bin.add(doc)
@@ -63,6 +67,9 @@ def test_serialize_doc_bin():
         assert doc.text == texts[i]
         assert doc.cats == cats
         assert len(doc.spans) == 1
+        assert doc.spans["start"][0].label_ == "UNUSUAL_SPAN_LABEL"
+        assert doc.spans["start"][0].id_ == "UNUSUAL_SPAN_ID"
+        assert doc.spans["start"][0].kb_id_ == "UNUSUAL_SPAN_KB_ID"
         assert doc[0].norm_ == "UNUSUAL_TOKEN_NORM"
         assert doc[0].ent_id_ == "UNUSUAL_TOKEN_ENT_ID"
 
diff --git a/spacy/tokens/_serialize.py b/spacy/tokens/_serialize.py
index c4e8f26f4..73c857d1f 100644
--- a/spacy/tokens/_serialize.py
+++ b/spacy/tokens/_serialize.py
@@ -124,6 +124,10 @@ class DocBin:
         for key, group in doc.spans.items():
             for span in group:
                 self.strings.add(span.label_)
+                if span.kb_id in span.doc.vocab.strings:
+                    self.strings.add(span.kb_id_)
+                if span.id in span.doc.vocab.strings:
+                    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 3bc404dd0..a54b4ad3c 100644
--- a/spacy/tokens/doc.pyx
+++ b/spacy/tokens/doc.pyx
@@ -1346,6 +1346,10 @@ cdef class Doc:
         for group in self.spans.values():
             for span in group:
                 strings.add(span.label_)
+                if span.kb_id in span.doc.vocab.strings:
+                    strings.add(span.kb_id_)
+                if span.id in span.doc.vocab.strings:
+                    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