diff --git a/portal/Makefile.am.inc b/portal/Makefile.am.inc
index 5cef72be..adea910f 100644
--- a/portal/Makefile.am.inc
+++ b/portal/Makefile.am.inc
@@ -37,4 +37,4 @@ CLEANFILES += $(portal_built_sources)
 
 flatpak_portal_LDADD = $(AM_LDADD) $(BASE_LIBS)
 flatpak_portal_CFLAGS = $(AM_CFLAGS) $(BASE_CFLAGS)
-flatpak_portal_CPPFLAGS = -I$(builddir)/portal
+flatpak_portal_CPPFLAGS = $(AM_CPPFLAGS) -I$(builddir)/portal