diff --git a/writeimg.c b/writeimg.c
index e48c1c3..8a445ce 100644
--- a/writeimg.c
+++ b/writeimg.c
@@ -207,6 +207,8 @@ int main(int argc, char *argv[]) {
     wjob.bufsize = BLOCKSIZE;
     wjob.block_size = BLOCKSIZE;
 
+    assert(file_stat.st_size >= 0);
+
     perform_write(&wjob);
 
     if (wjob.buffer)