diff --git a/scripts/serial_command.py b/scripts/serial_command.py
index 175a379..c7da679 100644
--- a/scripts/serial_command.py
+++ b/scripts/serial_command.py
@@ -98,7 +98,8 @@ def transfer(cli, firmware, *, dtb=False, kernel=False, rootfs=False, rootfs_siz
     if rootfs and len(image) > 0x1800000:
         raise RuntimeError("RootFS exceeds the non-overlapping RAM buffer capacity (0x1800000)")
     # Invalidate any previous transfer length before attempting a new transfer.
-    cli.command("setenv filesize")
+    cli.command("setenv filesize 0")
+    cli.command("printenv filesize", r"^filesize=0$")
     output = cli.command(
         f"tftp {address:#x} {filename}",
         r"^Bytes transferred = \d+", timeout=120,