Find a number between two Fibonacci series

def fib_interval(x):
    """ returns the largest fibonacci
    number smaller than x and the lowest
    fibonacci number higher than x"""
   
    old, new = 0, 1
    while True:
        if new < x:
            old, new = new, old+new
        else:
            if new == x: 
                new = old + new
            return (old, new)
        
            
while True:
    x = int(input("Your number: "))
    if x <= 0:
        break
    lub, sup = fib_interval(x)
    print("Largest Fibonacci Number smaller than x: " + str(lub))
    print("Smallest Fibonacci Number larger than x: " + str(sup))