F# type error with one level of indentation, not with another

I'm going through the Hopac documentation, and I came across this bizarre type error:

let create x = Job.delay <| fun () ->    let c = {reqCh = Ch (); replyCh = Ch ()}    Job.iterateServer x <| fun x ->        c.reqCh >>= function          | Get -> c.replyCh *<- x >>-. x          | Put x -> Job.result x>>-. cScrapers.SC.fsx(48,10): error FS0001: Type mismatch. Expecting a''a'but given a'Cell<'a>'The types ''a' and 'Cell<'a>' cannot be unified.

c is a Cell<'a>. This code compiles just fine with the code from the page:

let create x = Job.delay <| fun () ->  let c = {reqCh = Ch (); replyCh = Ch ()}  Job.iterateServer x <| fun x ->        c.reqCh >>= function          | Get -> c.replyCh *<- x >>-. x          | Put x -> Job.result x>>-. c

The only difference between these two is the depth of the first indent -- 2 as opposed to 4.

Normally, when there's a problem with indentation, I get a "Possibly incorrect indentation" warning -- not a type error. So it must be really confused.

I've been using F# for ten years now, and I still don't understand the indentation rules.

What am I missing?