TypeError: object of type 'NoneType' has no len()

def merge(lst1, lst2):
    """ merging two ordered lists using
        the three pointer algorithm """
    n1 = len(lst1)
    n2 = len(lst2)
    lst3 = [0 for i in range(n1 + n2)]  # alocates a new list
    i = j = k = 0  # simultaneous assignment

    while (i < n1 and j < n2):
        if (lst1[i] <= lst2[j]):
            lst3[k] = lst1[i]
            i = i +1
        else:
            lst3[k] = lst2[j]
            j = j + 1
        k = k + 1  # incremented at each iteration
    lst3[k:] = lst1[i:] + lst2[j:]  # append remaining elements

def multi_merge_v3(lst_of_lsts):
     m = len(lst_of_lsts)
     merged = []
     for i in range(m):
          merged= merge((merged),(lst_of_lsts)[i])
     return(merged)

for i in range(m):
      #Assigning None here
      merged = merge(merged, lst_of_lsts[i])
 return(merged)

return lst3