Releasing the result of newForeignPtr nullFunPtr nullPtr end in core dump, due to the fact that finalizer function is set to null pointer.

When I run something like this in GHCI:

> import System.Mem
> import Foreign.Ptr 
> import Foreign.ForeignPtr
> import System.IO.Unsafe
> import qualified Data.Map as Map
> a <- return $ Map.singleton 1 (unsafePerformIO $ newForeignPtr nullFunPtr nullPtr)
Loading package array-0.5.0.0 ... linking ... done.
Loading package deepseq-1.3.0.2 ... linking ... done.
Loading package containers-0.5.5.1 ... linking ... done.
> print a
fromList [(1,0x0000000000000000)]
> performGC
> ^D
Leaving GHCi.
[1]    3782 segmentation fault (core dumped)  ghci

it wont crash until exit from ghci which is correct. But if I do something similar but using let binding it will crash even if variable a didn't lose its scope

> import System.Mem
> import Foreign.Ptr 
> import Foreign.ForeignPtr
> import System.IO.Unsafe
> import qualified Data.Map as Map
> let a = Map.singleton 1 (unsafePerformIO $ newForeignPtr nullFunPtr nullPtr)
Loading package array-0.5.0.0 ... linking ... done.
Loading package deepseq-1.3.0.2 ... linking ... done.
Loading package containers-0.5.5.1 ... linking ... done.
> print a
fromList [(1,0x0000000000000000)]
[1]    3842 segmentation fault (core dumped)  ghci

Why is there a difference between doing it with do notation and with let binding?

I also expected that if I rebind variable a it will lose it's scope and will be released but it is not (see below)

> import System.Mem
> import Foreign.Ptr 
> import Foreign.ForeignPtr
> import System.IO.Unsafe
> import qualified Data.Map as Map
> a <- return $ Map.singleton 1 (unsafePerformIO $ newForeignPtr nullFunPtr nullPtr)
Loading package array-0.5.0.0 ... linking ... done.
Loading package deepseq-1.3.0.2 ... linking ... done.
Loading package containers-0.5.5.1 ... linking ... done.
> print a
fromList [(1,0x0000000000000000)]
> a <- return () -- rebinding varable a, it is no longer used
> performGC
>  -- no crash, varaible a not released