diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index c8bd1bd..b64c354 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -174,7 +174,7 @@
 
 #if defined(MBEDTLS_THREADING_C)
     if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR, ret ) );
 #endif
 
     ret = ssl_cookie_hmac( &ctx->hmac_ctx, *p - 4,
@@ -182,8 +182,8 @@
 
 #if defined(MBEDTLS_THREADING_C)
     if( mbedtls_mutex_unlock( &ctx->mutex ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR +
-                MBEDTLS_ERR_THREADING_MUTEX_ERROR );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR,
+                MBEDTLS_ERR_THREADING_MUTEX_ERROR ) );
 #endif
 
     return( ret );
@@ -210,7 +210,7 @@
 
 #if defined(MBEDTLS_THREADING_C)
     if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR + ret );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR, ret ) );
 #endif
 
     if( ssl_cookie_hmac( &ctx->hmac_ctx, cookie,
@@ -220,8 +220,8 @@
 
 #if defined(MBEDTLS_THREADING_C)
     if( mbedtls_mutex_unlock( &ctx->mutex ) != 0 )
-        return( MBEDTLS_ERR_SSL_INTERNAL_ERROR +
-                MBEDTLS_ERR_THREADING_MUTEX_ERROR );
+        return( MBEDTLS_ERROR_ADD( MBEDTLS_ERR_SSL_INTERNAL_ERROR ,
+                MBEDTLS_ERR_THREADING_MUTEX_ERROR ) );
 #endif
 
     if( ret != 0 )