diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 8dadbe1..33ea22a 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -77,6 +77,10 @@
 #error "MBEDTLS_DHM_C defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT) && !defined(MBEDTLS_SSL_TRUNCATED_HMAC)
+#error "MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT defined, but not all prerequisites"
+#endif
+
 #if defined(MBEDTLS_ECDH_C) && !defined(MBEDTLS_ECP_C)
 #error "MBEDTLS_ECDH_C defined, but not all prerequisites"
 #endif
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 0c51fea..b174d8a 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1184,6 +1184,22 @@
 #define MBEDTLS_SSL_TRUNCATED_HMAC
 
 /**
+ * \def MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
+ *
+ * Fallback to old, non-conforming implementation of the truncated
+ * HMAC extension which also truncates the HMAC key.
+ *
+ * \warning This should only be enabled temporarily when the use
+ *          of truncated HMAC is mandatory *and* the peer is an Mbed TLS
+ *          stack that doesn't use the fixed implementation yet.
+ *
+ * Uncomment to fallback to old, non-compliant truncated HMAC implementation.
+ *
+ * Requires: MBEDTLS_SSL_TRUNCATED_HMAC
+ */
+//#define MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
+
+/**
  * \def MBEDTLS_THREADING_ALT
  *
  * Provide your own alternate threading implementation.