Question about typecasting

So in this mission I am supposed to find numbers in the text and make a sum of them. I find them properly (still need to adjust it to see if there is a space after it or not) but I get an error when I try to convert the text.

I did try to debug it, it finds it correctly, no spaces, just the number, I cut it out of the main text even, still unable to convert the string to int, however doing something like

x = "a" a = int(a)

this works just fine, bellow is my super messy code(will polish it ofc once I can get past this thing), trying to find out what am I doing wrong but honestly idk whats wrong with it, I get an error on line 45, using pycharm : ValueError: invalid literal for int() with base 10: ''

text = "5 plus 6 is"

numbers = []

sum = 0

subtext = ""

start = 0

end = 0

aux = 0

x = len(text)

aux = int(sum)

print(aux+2)

for i in range(x):

    if text[i] >= "0" and text[i] <= "9":

        start = i

        end = 0

        for j in range(i,x):

            if text[j] >= "0" and text[j] <= "9":

                end += 1

            else:

                break

        print("*" + text[i:i+end:1] + "*")

        subtext = text[i:end:1]

        print(subtext)

        sum = int(subtext)

        numbers.append(int(sum))

    end = 0

print(numbers)