diff --git a/libavfilter/vf_extractplanes.c b/libavfilter/vf_extractplanes.c
index 111a132cad..6e7931b3e3 100644
--- a/libavfilter/vf_extractplanes.c
+++ b/libavfilter/vf_extractplanes.c
@@ -387,6 +387,7 @@ static av_cold int init_alphaextract(AVFilterContext *ctx)
     ExtractPlanesContext *s = ctx->priv;
 
     s->requested_planes = PLANE_A;
+    s->map[0] = 3;
 
     return 0;
 }