diff --git a/src/ctap/apdu.rs b/src/ctap/apdu.rs
index 66dde63..d9344ec 100644
--- a/src/ctap/apdu.rs
+++ b/src/ctap/apdu.rs
@@ -174,8 +174,10 @@ impl TryFrom<&[u8]> for APDU {
                 return Err(ApduStatusCode::SW_WRONG_LENGTH);
             }
 
-            let extended_apdu_le_len: usize =
-                payload.len().checked_sub(extended_apdu_lc + 3).unwrap_or(0);
+            let extended_apdu_le_len: usize = payload
+                .len()
+                .checked_sub(extended_apdu_lc + 3)
+                .unwrap_or(0xff);
             if extended_apdu_le_len > 3 {
                 return Err(ApduStatusCode::SW_WRONG_LENGTH);
             }