diff --git a/libavcodec/pnm_parser.c b/libavcodec/pnm_parser.c
index 58c3cd2431..29183f8f31 100644
--- a/libavcodec/pnm_parser.c
+++ b/libavcodec/pnm_parser.c
@@ -106,8 +106,10 @@ retry:
         if (next == END_NOT_FOUND)
             pnmpc->ascii_scan = sync - pnmctx.bytestream + skip;
     } else {
-        next = pnmctx.bytestream - pnmctx.bytestream_start + skip
-               + av_image_get_buffer_size(avctx->pix_fmt, avctx->width, avctx->height, 1);
+        int ret = av_image_get_buffer_size(avctx->pix_fmt, avctx->width, avctx->height, 1);
+        next = pnmctx.bytestream - pnmctx.bytestream_start + skip;
+        if (ret >= 0)
+            next += ret;
     }
     if (next != END_NOT_FOUND && pnmctx.bytestream_start != buf + skip)
         next -= pc->index;