diff --git a/library/ecdh.c b/library/ecdh.c
index 30c5f9f..eecae91 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -638,7 +638,7 @@
 
     *olen = ctx->grp.pbits / 8 + ( ( ctx->grp.pbits % 8 ) != 0 );
 
-    if( ctx->grp.id == MBEDTLS_ECP_DP_CURVE25519 )
+    if( mbedtls_ecp_get_type( &ctx->grp ) == MBEDTLS_ECP_TYPE_MONTGOMERY )
         return mbedtls_mpi_write_binary_le( &ctx->z, buf, *olen );
 
     return mbedtls_mpi_write_binary( &ctx->z, buf, *olen );