diff --git a/src/passes/1-parser/cameligo/dune b/src/passes/1-parser/cameligo/dune
index 786b20f31..95290beb8 100644
--- a/src/passes/1-parser/cameligo/dune
+++ b/src/passes/1-parser/cameligo/dune
@@ -40,4 +40,6 @@
 (executable
   (name Unlexer)
   (libraries str)
+  (preprocess
+    (pps bisect_ppx --conditional))
   (modules Unlexer))
diff --git a/src/passes/1-parser/reasonligo/dune b/src/passes/1-parser/reasonligo/dune
index 12b0c6d27..39250ecc8 100644
--- a/src/passes/1-parser/reasonligo/dune
+++ b/src/passes/1-parser/reasonligo/dune
@@ -44,4 +44,6 @@
 (executable
   (name Unlexer)
   (libraries str)
+  (preprocess
+    (pps bisect_ppx --conditional))
   (modules Unlexer))