diff --git a/libavcodec/truemotion1.c b/libavcodec/truemotion1.c
index b8d0de4686..da843c4440 100644
--- a/libavcodec/truemotion1.c
+++ b/libavcodec/truemotion1.c
@@ -645,7 +645,8 @@ static void truemotion1_decode_16bit(TrueMotion1Context *s)
         current_pixel_pair = (unsigned int *)current_line;
         vert_pred = s->vert_pred;
         mb_change_index = 0;
-        mb_change_byte = mb_change_bits[mb_change_index++];
+        if (!keyframe)
+            mb_change_byte = mb_change_bits[mb_change_index++];
         mb_change_byte_mask = 0x01;
         pixels_left = s->avctx->width;