diff --git a/src/wic/ksparser.py b/src/wic/ksparser.py
index 528a8180826f..dcf26f788083 100644
--- a/src/wic/ksparser.py
+++ b/src/wic/ksparser.py
@@ -132,10 +132,12 @@ def systemidtype(arg):
     """
     error = "Invalid system type: %s. must be hex "\
             "between 0x1 and 0xFF" % arg
-    try:
-        result = int(arg, 16)
-    except ValueError:
+    # int(arg, 16) also accepts a bare "82" (-> 130) and a leading sign
+    # like "+0x82"; the documented contract is an explicit 0x-prefixed
+    # hex byte, so require that form before converting.
+    if not re.fullmatch(r"0[xX][0-9a-fA-F]+", arg or ""):
         raise ArgumentTypeError(error)
+    result = int(arg, 16)
 
     if result <= 0 or result > 0xff:
         raise ArgumentTypeError(error)