diff --git a/libavformat/ast.c b/libavformat/ast.c
index 4f835401ed..5f69f47d44 100644
--- a/libavformat/ast.c
+++ b/libavformat/ast.c
@@ -26,8 +26,11 @@
 
 static int ast_probe(AVProbeData *p)
 {
-    if (AV_RL32(p->buf) == MKTAG('S', 'T', 'R', 'M'))
-        return AVPROBE_SCORE_MAX / 2;
+    if (AV_RL32(p->buf) == MKTAG('S','T','R','M') &&
+        AV_RB16(p->buf + 10) &&
+        AV_RB16(p->buf + 12) &&
+        AV_RB32(p->buf + 16))
+        return AVPROBE_SCORE_MAX / 3 * 2;
     return 0;
 }