Unexpected ' ' when converting from type string to type int

[Michael S via Digitalmars-d-learn]

Hello,  thanks for stopping in. I am fuddling through some 
exercises on a certain website, and I have come across a very 
frustrating bug I can't seem to fix.

The Problem:
Given a square matrix of size N×N, calculate the absolute 
difference between the sums of its diagonals.

Sample Input:
3
11 2 4
4 5 6
10 8 -12

First line is N, and the expected result of this example is 15.
If I hardcode in the value for N, this code works just peachy. 
It's when I am trying to actually read in the value of the first 
line from stdin that I am having a problem.

I seem to be unable to convert the string input to an integer to 
use later on. The code:

import std.stdio, std.math, std.string, std.conv;

void main(){
    auto matrix_size = readln;

    //below I have commented out the offending call to to!int and 
replaced it with a hardcoded value 3 in order to force it to work.
    auto ms = 3;//matrix_size.to!int;
    int[][] matrix = new int[][ms];
    for(int i = 0; i < ms; i++){
        string[] string_nums = readln.split;
        foreach (num; string_nums){
            auto value = num.to!int;
            matrix[i] ~= value;
        }
    }

    int primary_sum = 0;
    int secondary_sum = 0;
    int j = 0;
    int i = ms - 1;
    //determine diagonal sums
    for(int row = 0; row < ms; ++row){
        primary_sum += matrix[row][j];
        secondary_sum += matrix[row][i];
        ++j;
        --i;
    }
    auto result = abs(primary_sum - secondary_sum);
    result.writeln;
}