Could someone please tell me why I get a NullPointerException?

System.out.print("Enter the name of the file: ");
String fileName = readInput();

BufferedReader br = new BufferedReader(new FileReader(fileName + ".txt"));
String line;
int rowIndex = 0;
String matrix [][] = null;

while ((line = br.readLine()) != null) {
StringTokenizer st = new StringTokenizer(line);
int colIndex = 0;
while (st.hasMoreTokens()) {
matrix [rowIndex][colIndex++] = st.nextToken().trim();

}
}
rowIndex = rowIndex + 1;
br.close();
}