diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 77b091d..ad8754c 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -144,6 +144,7 @@
               MBEDTLS_SSL_EXT_MASK( ENCRYPT_THEN_MAC )                       | \
               MBEDTLS_SSL_EXT_MASK( EXTENDED_MASTER_SECRET )                 | \
               MBEDTLS_SSL_EXT_MASK( SESSION_TICKET )                         | \
+              MBEDTLS_SSL_EXT_MASK( TRUNCATED_HMAC )                         | \
               MBEDTLS_SSL_EXT_MASK( UNRECOGNIZED ) )
 
 /* RFC 8446 section 4.2. Allowed extensions for ClienHello */