Hello @Adebowale_Quadri_Ram! Welcome!
This error indicates that the function isn’t implemented correctly, but it is not uncommon. As part of the assignment and our own work, debugging incorrect code can be challenging, but there is also a way for to keep track of the progress of our code, so that we can know where goes wrong.
I suggest you to modify your function by adding printing lines to print the content of each variable used, and then run your modified function with some simple inputs. Please check out this post for a detailed example on how to do this, which is a general technique you can apply to this exercise and future ones.
Good luck with code debugging @Adebowale_Quadri_Ram! 
Raymond