memoize and apply...

var memoize3 = function(func) {

          var memoized = function(key) {
            if (!memoized.cache.hasOwnProperty(key)) memoized.cache[key] = func.apply(this, arguments);
            
            return memoized.cache[key]
          }

          memoized.cache = {};
          return memoized;
        }

//this works. But, I feel disingenuous using ".apply", because I don't fully understand it yet.
//vs, what I was trying to do:

var memoize3 = function(func) {

          var memoized = function(key) {
            var funcStore = func;
            if (!memoized.cache.hasOwnProperty(key)) memoized.cache[key] = funcStore(key);
            
            return memoized.cache[key]
          }

          memoized.cache = {};
          return memoized;
        }

/*------*/

Is there some rule against defining a function inside a function, and then running it inside of that function as a subroutine when called? Some syntax error I'm making?