diff --git a/aegisub/FFmpegSource2/Makefile.am b/aegisub/FFmpegSource2/Makefile.am
index f9cdfd92a..9b3a956a6 100644
--- a/aegisub/FFmpegSource2/Makefile.am
+++ b/aegisub/FFmpegSource2/Makefile.am
@@ -1,6 +1,6 @@
 noinst_LIBRARIES = libffmpegsource2_aegisub.a
 
-AM_CPPFLAGS = -I../src -D__UNIX__ -DHAVE_STRLCPY @LIBAVFORMAT_CFLAGS@ @LIBAVCODEC_CFLAGS@ @LIBSWSCALE_CFLAGS@ @LIBAVUTIL_CFLAGS@ @LIBPOSTPROC_CFLAGS@
+AM_CPPFLAGS = -I../src -D__UNIX__ -DFFMS_EXPORTS -DHAVE_STRLCPY @LIBAVFORMAT_CFLAGS@ @LIBAVCODEC_CFLAGS@ @LIBSWSCALE_CFLAGS@ @LIBAVUTIL_CFLAGS@ @LIBPOSTPROC_CFLAGS@
 
 
 libffmpegsource2_aegisub_a_SOURCES = \