diff --git a/satcfdi/pacs/swsapien.py b/satcfdi/pacs/swsapien.py
index ab4b784..42fad3c 100644
--- a/satcfdi/pacs/swsapien.py
+++ b/satcfdi/pacs/swsapien.py
@@ -144,7 +144,7 @@ def recover(self, document_id: str, accept: Accept = Accept.XML) -> Document:
         )
 
     def cancel(self, cfdi: CFDI, reason: CancelReason, substitution_id: str = None, signer: Signer = None) -> CancelationAcknowledgment:
-        document_id = cfdi["Complemento"]["TimbreFiscalDigital"]["UUID"]
+        document_id = cfdi["Complemento"]["TimbreFiscalDigital"]["UUID"].upper()
         rfc = cfdi["Emisor"]["Rfc"]
         path = f"cfdi33/cancel/{rfc}/{document_id}/{reason.value}"
         if substitution_id: