diff --git a/ble/att.h b/ble/att.h
index ea021a44f..e5a1dd6e7 100644
--- a/ble/att.h
+++ b/ble/att.h
@@ -105,6 +105,7 @@ extern "C" {
 #define ATT_ERROR_UNSUPPORTED_GROUP_TYPE           0x10
 #define ATT_ERROR_INSUFFICIENT_RESOURCES           0x11
 // custom BTstack ATT error coders
+#define ATT_ERROR_DATA_MISMATCH                    0x7e
 #define ATT_ERROR_TIMEOUT                          0x7F