diff --git a/disco.py b/disco.py
index 8f99f90..0ef0365 100644
--- a/disco.py
+++ b/disco.py
@@ -204,8 +204,9 @@ def generate_barcode(upc: str, dest_dir: Path) -> Path | None:
     digits = re.sub(r"\D", "", upc)
     if not digits:
         return None
-    # UPC-A needs exactly 11 digits (12th is check digit, auto-calculated)
-    digits = digits[-11:].zfill(11)
+    # UPC-A: pass first 11 digits, library auto-calculates the 12th (check digit)
+    # A full UPC is 12 digits where the 12th is already the check digit
+    digits = digits[:11].zfill(11)
     try:
         upc_cls = barcode.get_barcode_class("upca")
         bc = upc_cls(digits, writer=ImageWriter())