Natural forced alteration not played on midi file

[LEDOCQ-BOCCART]

Hi,

I 've written a piece in F major which contains b bemol notes an b 
natural ones (using Lilypond 2.10.23)
The Midi generated file - simply activated in the following code -

\midi {
}

plays the piece OK except for the b natural which are played b bemol 
i.e. 1/2 tone too low.

Is there a way to correct to get the correct tune?

Thanks for answering

Charlie