Import error on django models.py

I wrote this funcion on a utils.py located on the app direcroty:

from bm.bmApp.models import Client

def get_client(user):
    try:
        client = Client.objects.get(username=user.username)
    except Client.DoesNotExist:
        print "User Does not Exist"
        return None
    else:       
        return client

def to_safe_uppercase(string):
    if string is None:
        return ''
    return string.upper()

Then when i use the function to_safe_uppercase on my models.py file, by importing it in this way:

from bm.bmApp.utils import to_safe_uppercase 

I got the python error:

     from bm.bmApp.utils import to_safe_uppercase
ImportError: cannot import name to_safe_uppercase

I got the solution for this problem when i change the import statement for:

from bm.bmApp.utils import *

But i can't understand why is this, why when i import the specific function i got the error?