F# random class throws BadImageFormatException when calling .NextDouble() -

i have made small application book f#, when call rand.nextdouble() method throws badimageformatexception, @ first glance looks it's images have no idea i'm supposed here.

i'm using vs community 2013 using visual f# 2013. have 64 bit windows 7

i know it's nextdouble method that's problem through trying seperate let

let create_field num_asteroids =     let lerp (x: float<'u>) (y:float<'u>) (a:float) =         x*a+y*(1.0-a)      let rand = random()      [for =1 num_asteroids         let m =             (lerp earth_mass moon_mass (rand.nextdouble()))*             1.0e-4         let x = lerp 0.0<m> field_size (rand.nextdouble())         let y = lerp 0.0<m> field_size (rand.nextdouble())         let vx = max_velocity*(rand.nextdouble()*2.0-1.0)*0.1         let vy = max_velocity*(rand.nextdouble()*2.0-1.0)*0.1         yield             {             position = { x = x; y = y }             velocity = { x = vx; y = vy }             mass = m             name = "a"             }     ] 

this errorcode:

system.badimageformatexception unhandled message: unhandled exception of type 'system.badimageformatexception' occurred in asteroid_field_1.exe additional information: attempt made load program incorrect format. (exception hresult: 0x8007000b)

msdn tells me might have dll file not being loaded in correct format, don't know start.

http://pastebin.com/7lhchr3c

this full code

i tried in fsi.exe , works there fine

i fixed issue checking "optimize code" in properties of project in visual studio. got fix fellow student.