runtime and type encoding confusion on 64-bit

I am trying to understand what's going wrong in a weird class for  
which I'm trying to use runtime functions to access ivars (as the  
class doesn't define accessors). The context is a 64-bit executable  
(which is relevant).

The class has an ivar called (for this argument) "count" of type  
"unsigned int" (ivar_getTypeEncoding() returns "I" and also class-dump  
shows it as "unsigned int", yes I'm talking about arch=x86_64), so I'm  
trying to get the value as follows:

unsigned int count;
object_getInstanceVariable(obj, "count", (void *)&count);

This crashes with an "EXC_BAD_ACCESS"  signal.

However when I do the following it works:

NSUInteger count;
object_getInstanceVariable(obj, "count", (void *)&count);

This makes no sense to me, as this is the wrong type.

Does anyone understand why this can work when I do it wrong and does  
not work when I do it right?

I do know that the type of the ivar probably should be NSUInteger=long  
long for 64 bits, but that's not my fault (it's not my class).

Thanks,
Christiaan