diff --git a/src/util.py b/src/util.py
index 872f699..b8692e0 100644
--- a/src/util.py
+++ b/src/util.py
@@ -149,6 +149,8 @@ def extract(self, src, dest):
         self.pkg.extract(src, dest)
 
     def fdcopy(self, sfd, dfd, size=None):
+        if size is not None and size % (4 * 1024) != 0:
+            raise Exception("The size of the rootfs image file must be a multiple of 4KiB.")
         BLOCK = 16 * 1024 * 1024
         copied = 0
         bps = 0