diff --git a/pyproject.toml b/pyproject.toml
index 1eeece1b..620bd452 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -14,7 +14,7 @@ dependencies = [
   "numpy",
   "crcmod",
   "tqdm",
-  "pycapnp",
+  "pycapnp==2.1.0",
   "pycryptodome",
 ]