diff --git a/libavcodec/mjpegdec.c b/libavcodec/mjpegdec.c
index d78b5453a5..cfeca6a586 100644
--- a/libavcodec/mjpegdec.c
+++ b/libavcodec/mjpegdec.c
@@ -2793,8 +2793,8 @@ the_end:
                 dst[0][j] = g*257 >> 16;
                 dst[1][j] = b*257 >> 16;
                 dst[2][j] = r*257 >> 16;
-                dst[3][j] = 255;
             }
+            memset(dst[3], 255, w);
         }
     }
     if (s->adobe_transform == 2 && s->avctx->pix_fmt == AV_PIX_FMT_YUVA444P) {
@@ -2816,8 +2816,8 @@ the_end:
                 dst[0][j] = r*257 >> 16;
                 dst[1][j] = (g*257 >> 16) + 128;
                 dst[2][j] = (b*257 >> 16) + 128;
-                dst[3][j] = 255;
             }
+            memset(dst[3], 255, w);
         }
     }