diff --git a/library/ssl_tls13_keys.h b/library/ssl_tls13_keys.h
index 31ffe44..14f6e48 100644
--- a/library/ssl_tls13_keys.h
+++ b/library/ssl_tls13_keys.h
@@ -60,8 +60,9 @@
     mbedtls_ssl_tls13_labels.LABEL,              \
     MBEDTLS_SSL_TLS1_3_LBL_LEN(LABEL)
 
-#define MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN  \
-    sizeof(union mbedtls_ssl_tls13_labels_union)
+/* Maximum length of the label field in the HkdfLabel struct defined in
+ * RFC 8446, Section 7.1, excluding the "tls13 " prefix. */
+#define MBEDTLS_SSL_TLS1_3_HKDF_LABEL_MAX_LABEL_LEN 249
 
 /* The maximum length of HKDF contexts used in the TLS 1.3 standard.
  * Since contexts are always hashes of message transcripts, this can