dict_1 = {1: {2}, 2: {3}, 3: {4}, 4: {11}}

dict_2 = {1: {2}, 2: {1}, 3: {4}, 4: {1,3}}

to get:

dict_1 = {1: {2}, 2: {3,1}, 3: {4}, 4: {11,1,3}}

How would I add the values from dict_2 in dict_1 without deleting the ones already there? I can add them as a list but for some reason, I get the error list unhashable when trying to add them:

to_add = set()
 for x in dict_2:
  if x in dict_1.keys():
  to_add.add(dict_1[x])
  to_add.add(dict_2[x])
  dict_1[x] = to_add