Extract source code from APK file not working. ".. ROOT cause" message

Question

I've recently begun programming.

I've been following this tutorial to extract a source code from my APK file on my MacBook Pro:

https://www.youtube.com/watch?v=-AX4NYE-9V8

All the steps worked well until I tried the command "sh dex2jar.sh classes.dex". My Terminal gives me this message:

 dex2jar version: translator-0.0.9.8
dex2jar classes.dex -> classes_dex2jar.jar
. while process file: [classes.dex]
.. ROOT cause:
java.lang.StringIndexOutOfBoundsException: String index out of range: 2
    at java.lang.String.checkBounds(String.java:375)
    at java.lang.String.<init>(String.java:535)
    at com.googlecode.dex2jar.reader.DexFileReader.readDex(DexFileReader.java:122)
    at com.googlecode.dex2jar.reader.DexFileReader.readDex(DexFileReader.java:149)
    at com.googlecode.dex2jar.v3.Main.doFile(Main.java:63)
    at com.googlecode.dex2jar.v3.Main.main(Main.java:85)
Done.

I do not get a "classes_dex2jar.jar" file.

What's happening?