diff --git a/libavformat/swfdec.c b/libavformat/swfdec.c
index ca2a1c458b..528bc236f2 100644
--- a/libavformat/swfdec.c
+++ b/libavformat/swfdec.c
@@ -289,6 +289,7 @@ static int swf_read_packet(AVFormatContext *s, AVPacket *pkt)
             const int bmp_fmt = avio_r8(pb);
             const int width   = avio_rl16(pb);
             const int height  = avio_rl16(pb);
+            int pix_fmt;
 
             len -= 2+1+2+2;
 
@@ -367,7 +368,7 @@ static int swf_read_packet(AVFormatContext *s, AVPacket *pkt)
 
             switch (bmp_fmt) {
             case 3:
-                st->codec->pix_fmt = AV_PIX_FMT_PAL8;
+                pix_fmt = AV_PIX_FMT_PAL8;
                 for (i = 0; i < colormapsize; i++)
                     if (alpha_bmp)  colormap[i] = buf[3]<<24 | AV_RB24(buf + 4*i);
                     else            colormap[i] = 0xffU <<24 | AV_RB24(buf + 3*i);
@@ -379,14 +380,20 @@ static int swf_read_packet(AVFormatContext *s, AVPacket *pkt)
                 memcpy(pal, colormap, AVPALETTE_SIZE);
                 break;
             case 4:
-                st->codec->pix_fmt = AV_PIX_FMT_RGB555;
+                pix_fmt = AV_PIX_FMT_RGB555;
                 break;
             case 5:
-                st->codec->pix_fmt = alpha_bmp ? AV_PIX_FMT_ARGB : AV_PIX_FMT_0RGB;
+                pix_fmt = alpha_bmp ? AV_PIX_FMT_ARGB : AV_PIX_FMT_0RGB;
                 break;
             default:
                 av_assert0(0);
             }
+            if (st->codec->pix_fmt != AV_PIX_FMT_NONE && st->codec->pix_fmt != pix_fmt) {
+                av_log(s, AV_LOG_ERROR, "pixel format change unsupported\n");
+                res = AVERROR_PATCHWELCOME;
+                goto bitmap_end;
+            }
+            st->codec->pix_fmt = pix_fmt;
 
             if (linesize * height > pkt->size) {
                 res = AVERROR_INVALIDDATA;