diff --git a/tools/target_dem_fuzzer.c b/tools/target_dem_fuzzer.c
index 409599636d..cc097da0d7 100644
--- a/tools/target_dem_fuzzer.c
+++ b/tools/target_dem_fuzzer.c
@@ -70,6 +70,8 @@ static int64_t io_seek(void *opaque, int64_t offset, int whence)
         if (offset > INT64_MAX - c->filesize)
             return -1;
         offset += c->filesize;
+    } else if (whence == AVSEEK_SIZE) {
+        return c->filesize;
     }
     if (offset < 0 || offset > c->filesize)
         return -1;