diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 7974b27..1de31e8 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -5268,6 +5268,11 @@
 #define SSL_CURVE_X25519 SSL_GROUP_X25519
 #define SSL_CURVE_X25519_KYBER768_DRAFT00 SSL_GROUP_X25519_KYBER768_DRAFT00
 
+// TLSEXT_nid_unknown is a constant used in OpenSSL for
+// |SSL_get_negotiated_group| to return an unrecognized group. BoringSSL never
+// returns this value, but we define this constant for compatibility.
+#define TLSEXT_nid_unknown 0x1000000
+
 
 // Compliance policy configurations
 //