Traceback (most recent call last):
File "/run-1341144766-1067082874/solution.py", line 27, in 
main()
File "/run-1341144766-1067082874/solution.py", line 11, in main
if len(s[i:j+1]) > 0:
MemoryError
Error in sys.excepthook:
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/apport_python_hook.py", line 64, in apport_excepthook
from apport.fileutils import likely_packaged, get_recent_crashes
File "/usr/lib/python2.7/dist-packages/apport/__init__.py", line 1, in 
from apport.report import Report
MemoryError

Original exception was:
Traceback (most recent call last):
File "/run-1341144766-1067082874/solution.py", line 27, in 
main()
File "/run-1341144766-1067082874/solution.py", line 11, in main
if len(s[i:j+1]) > 0:
MemoryError

当我尝试运行以下程序时，出现了以上错误。有人可以解释什么是内存错误，以及如何解决此问题？。
该程序将字符串作为输入，并找到所有可能的子字符串，并从中创建一个集（按字典顺序），并应在用户要求的相应索引处打印值，否则应打印“无效”

def main():
    no_str = int(raw_input())
    sub_strings= []
    for k in xrange(0,no_str):
        s = raw_input()
        a=len(s)
        for i in xrange(0, a):
            for j in xrange(0, a):
                if j >= i:
                    if len(s[i:j+1]) > 0:
                        sub_strings.append(s[i:j+1])
    sub_strings = list(set(sub_strings))
    sub_strings.sort()
    queries= int(raw_input())
    resul = []
    for i in xrange(0,queries):
        resul.append(int(raw_input()))
    for p in resul:
        try:
            print sub_strings[p-1]
        except IndexError:
            print 'INVALID'


if __name__ == "__main__":
   main()