Assigning specific type object into an generic array

I have a generic type called Vector<T>, I created it as so, cause the T might be float or Complex :

public class Vector<T>
    {
        #region Properties
        public ulong Length
        {
            get
            {
                return _length;
            }
        }
        public VectorType VectorType
        {
            get
            {
                return _vectorType;
            }
        }

        #endregion

        #region Indexers
        public T this[ulong index]
        {
            get
            {
                return _data[index];
            }
            set
            {
                _data[index] = value;
            }
        }

        #endregion

        #region Constructors

        public Vector(VectorType vectorType, T[] data)
        {
            if (!((data is float[]) || (data is Complex[])))
            {
                throw new InvalidDataException("Data must be array of float or array of Complex");
            }
            _data = new T[_length = (ulong)data.Length];
            for (ulong i = 0; i < _length; i++)
            {
                _data[i] = data[i];
            }
            _vectorType = vectorType;
        }


        public Vector(VectorType vectorType, Vector<T> vector)
        {
            _data = new T[_length = vector.Length];
            for (ulong i = 0; i < _length; i++)
            {
                _data[i] = vector[i];
            }
            _vectorType = vectorType;
        }

        #endregion

        #region Methods

            //Unity Matrix, this vector has 1/N everywhere
            public static Vector<float> e(VectorType vectorType, ulong length)
            {
                var data = new float[length];
                for (ulong i = 0; i < length; i++)
                {
                    data[i] = (float)1 / length;
                }
                var vectorE = new Vector<float>(vectorType, data); 
                return vectorE;
            }

            public float Sum()
            {
                float sum = 0;
                if (_data is float[])
                {
                    sum = (_data as float[]).Sum();
                }
                else
                {
                    if (_data is Complex[])
                    {
                        for (ulong i = 0; i < _length; i++)
                        {
                            sum += (float)
                                Math.Sqrt(Math.Pow((_data[i] as Complex?).Value.Real, 2) +
                                          Math.Pow((_data[i] as Complex?).Value.Imaginary, 2));
                        }
                    }
                }
                return sum;
            }

            public bool CheckIfSochasitc()
            {
                return Math.Abs(Sum() - 1) < float.Epsilon;
            }

            public void Normalize()
            {
                var sum = Sum();

                if (_data is float[])
                {
                    for (ulong i = 0; i < _length; i++)
                    {
                        float x = ((float) _data[i])/sum;
                        _data[i] =  (T)x;
                    }
                }

            }

        #endregion

        #region Operators
        //I omitted the code inhere to avoid overload
        #endregion

        #region Fields

            private  ulong _length;
            private readonly VectorType _vectorType;
            private T[] _data;

        #endregion
    }

    public enum VectorType
    {
        Row,Column        
    }

My problem is that I have a generic array (if I can call it so) :

private T[] _data;

And I have the Normalize() method:

public void Normalize()
          {
             var sum = Sum();
             if (_data is float[])
             {
                for (ulong i = 0; i < _length; i++)
                {
                    //Here is the problem
                    _data[i] =  ((_data[i] as float?) / sum);
                }
             }
          }

This doesn't work saying can't cast float to T tried to search but couldn't find helpful aide, any clarification I'd be thankful.

Update :

The Sum() method always returns a float