How to fix this C++ substr out of bounds error

I am a beginner C++ programmer.

I have this array declared somewhere:

char* patterns[10000];

And a C++ String as input to a function. I am then extracting out all substrings of length 20 (overlapping). I am converting them to C style strings for performance.

for(int i = 0; i<=input.length()-20;i++){
    patterns[i] = strdup(input.substr(i,20).c_str());
    count = i;
}

However, I get the following error

    terminate called after throwing an instance of 'std::out_of_range'
what(): basic_string::substr

The odd thing is, even when the I use (1,20) as constant arguments for substr, the error still shows up at some point during the loop. Furthermore, simply printing the substring in a loop is fine. Something must be occurring with either strdup or the assignment to the array, but I don't know what could be happening.