diff --git a/libass/Makefile.am b/libass/Makefile.am
index 98d763137..c76c0ecdd 100644
--- a/libass/Makefile.am
+++ b/libass/Makefile.am
@@ -1,6 +1,6 @@
 noinst_LIBRARIES = libass_aegisub.a
 
-libass_aegisub_a_CPPFLAGS = @FREETYPE_CFLAGS@ -DHAVE_FONTCONFIG -DUSE_ICONV @FONTCONFIG_CFLAGS@
+AM_CPPFLAGS = @FREETYPE_CFLAGS@ -DHAVE_FONTCONFIG -DUSE_ICONV @FONTCONFIG_CFLAGS@
 
 libass_aegisub_a_SOURCES = \
 	ass.c \