Function returns invalid literal for int() with base 10: ''

I have a function, that parses string and takes values by using indexes. When I try to take the exact variable and convert it into integer, it gives me a ValueError. The exact code part is:

y = special_string[y_start:y_end]
print(y)
y = int(y)

Traceback for it:

14

Traceback (most recent call last):
  File "levelBuilder.py", line 255, in <module>
    newLevel, rows, columns = importLevel(levelName)
  File "levelBuilder.py", line 124, in importLevel
    y = int(y)
ValueError: invalid literal for int() with base 10: ''

The question is - what can cause this error? As you see, y is integer (14). When I use the same method for another variable (x), it works correctly