AFL taking input from Stdin

I am trying to Fuzz a binary file that takes input from the user(Stdin). When I try Afl-fuzz and then my binary something like

afl-fuzz a.out

It asks for the required parameters that are specifying the input and output directories.

afl-fuzz -i input_dir -o output_dir a.out 

It gives the error Looks like there are no valid test cases in the input directory. If I provide a sample test case and try again it starts fuzzing but doesn't ask for input from the user. I am a total noob in this field so any kind of help would be appreciated.