diff --git a/flake.nix b/flake.nix
--- a/flake.nix
+++ b/flake.nix
@@ -284,6 +284,9 @@
               rm -f api/tangled/*
               lexgen --build-file lexicon-build-config.json lexicons
               sed -i.bak 's/\tutil/\/\/\tutil/' api/tangled/*
+              # lexgen generates incomplete Marshaler/Unmarshaler for union types
+              find api/tangled/*.go -not -name "cbor_gen.go" -exec \
+                sed -i '/^func.*\(MarshalCBOR\|UnmarshalCBOR\)/,/^}/ s/^/\/\/ /' {} +
               ${pkgs.gotools}/bin/goimports -w api/tangled/*
               go run ./cmd/cborgen/
               lexgen --build-file lexicon-build-config.json lexicons