Getting Type Error

I am trying to write matrix multiplication programme.But I am getting error called "Type Error:Int Object is not iterable".This is my code

def matmult(m1,m2):
  r1=[]
  r2=[]
  for a in range(len(m2[0])):
    r1.append(0)
  for b in range(len(m1)):
    r2.append(r1)
  for i in range(len(m1)):
    for j in range(len(m2[0])):
      for k in range(len(m2)):
        r2 += m1[i][k]+m2[k][j]

I am getting error at last line of code.What could be the cause?

Copyright Notice：Content Author:「srinivas chaitanya」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/42242581/getting-type-error