diff --git a/requirements.txt b/requirements.txt
index 3fbf9a3..a42cae4 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,6 @@
 tensorflow >= 2.5.0
 numpy < 1.24.0  # XXX https://github.com/OCR-D/ocrd_calamari/issues/87
+protobuf < 4.0  # XXX https://github.com/OCR-D/ocrd_calamari/issues/89
 calamari-ocr == 1.0.*
 setuptools >= 41.0.0  # tensorboard depends on this, but why do we get an error at runtime?
 click