diff --git a/lib/roken/Makefile.am b/lib/roken/Makefile.am
index 920912611..48c2b7a0f 100644
--- a/lib/roken/Makefile.am
+++ b/lib/roken/Makefile.am
@@ -10,7 +10,7 @@ lib_LTLIBRARIES = libroken.la
 libroken_la_LDFLAGS = -version-info 19:0:1
 libroken_la_CPPFLAGS = -DBUILD_ROKEN_LIB
 
-AM_CPPFLAGS = $(libroken_la_CPPFLAGS)
+AM_CPPFLAGS += $(libroken_la_CPPFLAGS)
 
 noinst_PROGRAMS = make-roken snprintf-test resolve-test