diff --git a/src/fido/management.c b/src/fido/management.c
index b6ac61e..7be2e63 100644
--- a/src/fido/management.c
+++ b/src/fido/management.c
@@ -85,6 +85,7 @@ int man_get_config() {
     res_APDU[res_APDU_size++] = TAG_SERIAL;
     res_APDU[res_APDU_size++] = 4;
     memcpy(res_APDU + res_APDU_size, pico_serial.id, 4);
+    res_APDU[res_APDU_size] &= ~0xFC; // Force 8-digit serial number
     res_APDU_size += 4;
     res_APDU[res_APDU_size++] = TAG_FORM_FACTOR;
     res_APDU[res_APDU_size++] = 1;
diff --git a/src/fido/otp.c b/src/fido/otp.c
index 0e41dab..e434d27 100644
--- a/src/fido/otp.c
+++ b/src/fido/otp.c
@@ -476,6 +476,7 @@ int cmd_otp() {
     }
     else if (p1 == 0x10) {
         memcpy(res_APDU, pico_serial.id, 4);
+        res_APDU[0] &= ~0xFC; // Force 8-digit serial number
         res_APDU_size = 4;
     }
     else if (p1 == 0x13) { // Get config