diff --git a/libavformat/utils.c b/libavformat/utils.c
index 7940037782..be73c4acb7 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -459,7 +459,7 @@ int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt,
 {
     AVProbeData pd = { filename ? filename : "", NULL, -offset };
     unsigned char *buf = NULL;
-    int ret = 0, probe_size;
+    int ret = 0, probe_size, buf_offset = 0;
 
     if (!max_probe_size) {
         max_probe_size = PROBE_BUF_MAX;
@@ -499,7 +499,7 @@ int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt,
             score = 0;
             ret = 0;            /* error was end of file, nothing read */
         }
-        pd.buf_size += ret;
+        pd.buf_size = buf_offset += ret;
         pd.buf = &buf[offset];
 
         memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);