getElementById results to null: Uncaught TypeError: Cannot read property 'getAttribute' of null

Uncaught TypeError: Cannot read property 'getAttribute' of null
The error line is shown in the third line of this code.

Code

vara=param.getAttribute("a");
varb = document.getElementById(a);
varc_class = b.getAttribute("class");

I am using jquery.

I wonder what it is because only certain users have errors.
What does this mean?
Is it correct that the contents of the getAttribute function in the third line are null?

I modified the code a little, but it looks like this.