diff --git a/library/rsa.c b/library/rsa.c
index fea76bf..f4bec46 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1085,7 +1085,7 @@
     mbedtls_mpi_free( &I );
 
     if( ret != 0 && ret >= -0x007f )
-        return( MBEDTLS_ERR_RSA_PRIVATE_FAILED + ret );
+        return( MBEDTLS_ERR_ADD( MBEDTLS_ERR_RSA_PRIVATE_FAILED, ret ) );
 
     return( ret );
 }