Type conversion in Python 2.7 vs 3.5, how to make my 2.7 code work under 3.5?

I’m porting 2.7 code into 3.5 and the 2.7 code works just fine. After running 2to3 and changing the obvious in the old code, I still get the following error message:

Exception in thread Thread-7:
Traceback (most recent call last):
  File "C:UsersAnaconda3libthreading.py", line 914, in _bootstrap_inner
    self.run()
  File "C:UsersAnaconda3libthreading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File "<ipython-input-2-eadc3d59ae9f>", line 70, in receiveData
    nftp = self.parseNFTPData(stringBuffer(data[:-1]))
  File "<ipython-input-2-eadc3d59ae9f>", line 101, in parseNFTPData
    current_string += c
TypeError: Can't convert 'int' object to str implicitly

I do understand the error, but why is this error raised in under Python 3 and not 2?

Code as follows:

def parseNFTPData(self, strbuf):
      quote = False
      result = []
      current_string = ""
   
      while not strbuf.eof(): 
         c = strbuf.read()
         if c == """:
            if quote: 
               quote = False
            else:
               quote = True
         elif c == "(" and not quote:
            list = self.parseNFTPData(strbuf)
            result.append(list)
         elif c == ")" and not quote: 
            result.append(self.typeConvert(current_string))
            strbuf.read()
            return result
         elif len(current_string) > 0 and c == " " and not quote:
            result.append(self.typeConvert(current_string))
            current_string = ""
         else: 
            current_string += c
   
      if len(current_string) > 0: 
         result.append(self.typeConvert(current_string))
      return result

strbuf is of class stringBuffer:

class stringBuffer:
   def __init__(self, string):
      self.buffer  = string
      self.readptr = 0
   
   def read(self): 
      if self.readptr < len(self.buffer): 
         readc = self.buffer[self.readptr]
         self.readptr += 1
      else:
         readc = False
      return readc

   def eof(self): 
      if self.readptr >= len(self.buffer): 
         return True
      else:
         return False 

   def peek(self): 
      return self.buffer[self.readptr]