diff --git a/master.Makefile.in b/master.Makefile.in
index 134bd1c..51a3bf4 100644
--- a/master.Makefile.in
+++ b/master.Makefile.in
@@ -35,7 +35,7 @@ OCAMLBEST= @OCAMLBEST@
 OCAMLVERSION = @OCAMLVERSION@
 EXTRAC_CRC = $(OCAMLLIB)/extract_crc
 OCAMLDOC= @OCAMLDOC@
-OCAMLPP=-pp 'grep -v DEBUG'
+OCAMLPP=-pp 'grep -av DEBUG'
 
 # For installation
 ##############