Why is it a memory ERRO.

[Dsby via Digitalmars-d-learn]

the Code:
class MyClass
{
	this(){
		by = new ubyte[10000];
		++i;
	}
	~this(){
		GC.free(by.ptr);
		by = null;
		writeln("free");
	}
	void show(){
		writeln(i);
	};
private:
	ubyte[]   by;
	static i = 0;
};

void main()
{
	bool start = true;
	int i = 0;
	while(start){
		auto obj = new MyClass();
		obj.show();
		Thread.sleep(5.msecs);
		//obj.destroy;
		//GC.free(cast(void *)obj);
		++i;
		if (i > 20000)
			break;
	}
}

the code will be :
341
core.exception.InvalidMemoryOperationError at src/core/exception.d(679): Invalid memory operation
----------------
core.exception.InvalidMemoryOperationError at src/core/exception.d(679): Invalid memory operation
----------------

.why is it?
if < obj.destroy; > is exec. the code will not errno.