diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
index 2c5fcc0532..6bcd7b94d2 100644
--- a/fftools/ffmpeg.c
+++ b/fftools/ffmpeg.c
@@ -1266,7 +1266,8 @@ static void do_video_out(OutputFile *of,
             ost->forced_keyframes_expr_const_values[FKF_N] += 1;
         } else if (   ost->forced_keyframes
                    && !strncmp(ost->forced_keyframes, "source", 6)
-                   && in_picture->key_frame==1) {
+                   && in_picture->key_frame==1
+                   && !i) {
             forced_keyframe = 1;
         }