void pointer to struct pointer cast

I declared a void pointer

void *ptr;

and a typedef struct named testStructure, with a member 'unsigned int a'.

Now I will cast this pointer to a struct pointer:

ptr=(testStructure*)malloc(sizeof(testStructure));

Now I try to access the member of the struct...

ptr->a=5;

...and I get an error "expression must have a pointer-to-struct-or-union type"...
Why is 'ptr' still void? It seems, a void pointer cannot be casted to any other pointer type...
How I can do this properly?
I just need ONE pointer, which can point to different structures, that means, I need to cast a void pointer...