diff --git a/src/cbor/tests/dune b/src/cbor/tests/dune
index 2a76380d..7ae1aeb4 100644
--- a/src/cbor/tests/dune
+++ b/src/cbor/tests/dune
@@ -15,6 +15,9 @@
 (executable
  (name t)
  (modules t)
+ (preprocess
+  (action
+   (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
  (libraries qcheck-core qcheck-core.runner containers containers.cbor))
 
 (rule