diff --git a/extensions/pknock/Makefile.am b/extensions/pknock/Makefile.am
index 18342eb..a7273ad 100644
--- a/extensions/pknock/Makefile.am
+++ b/extensions/pknock/Makefile.am
@@ -1,5 +1,8 @@
 # -*- Makefile -*-
 
+AM_CPPFLAGS = ${regular_CPPFLAGS}
+AM_CFLAGS   = ${regular_CFLAGS}
+
 include ../../Makefile.extra
 
 noinst_PROGRAMS = pknlusr