diff --git a/src/wic/ksparser.py b/src/wic/ksparser.py
index 8e5fbfadd81f..24c7015b336d 100644
--- a/src/wic/ksparser.py
+++ b/src/wic/ksparser.py
@@ -68,6 +68,8 @@ def sizetype(default, size_in_bytes=False):
             except ValueError:
                 raise ArgumentTypeError("Invalid size: %r" % arg)
 
+        if size < 0:
+            raise ArgumentTypeError("Invalid size: %r" % arg)
 
         if size_in_bytes:
             if suffix == 's' or suffix == 'S':