Trying to build an uppercase to lowercase string converter in Python 3.7, this is my code:

```
def convertChar(char):
    if char >= 'A' and char <= 'Z':
        return chr(ord(char) + 32)
def toLowerCase(string):
    for char in string:
        string = string + convertChar(char)
    return string
string = input("Enter string - ")
result = toLowerCase(string)
print(result)

```

And this is my output:

```
Enter string - HELlo
Traceback (most recent call last):
  File "loweralph.py", line 11, in <module>
    string = toLowerCase(result)
  File "loweralph.py", line 6, in toLowerCase
    result = result + convertChar(char)
TypeError: can only concatenate str (not "NoneType") to str

```

I am really new to Python and I saw answers for 'list to list' TypeErrors and [one other answer](https://stackoverflow.com/questions/37556886/getting-typeerror-when-trying-to-concatenate-str-and-nonetype-objects) for str to str, but I couldn't relate it to my code. If somebody could explain what I'm doing wrong, it would be great!