Debug assertion error when deleting class

[HairyDave]

I have a dll that contains a global variable. This global variable is a class:

MyClass *mcClass

In an exported function I call:

 mcClass = new MyClass(...);

which obviously constructs the class. However, I then wanted to add the ability to call that function, but if the var mcClass has already been initialised, I wanted it to delete it first. So, in DllMain I placed:

mcClass = NULL;

On the call to my function it would therefore do:

if(mcClass != NULL)
  delete mcClass;
mcClass = new MyClass;

However, when this executes in Debug mode, it causes an Assertion Error:

Expression: _CrtIsValidHeapPointer(pUserData)

Why is this the case? Surely if the class is global, it can be deleted wherever?

Thanks

HD