Simple Subtraction Problem

Attempting to do the death-defying performance of simple subtraction between two columns in Power Query is not giving me the expected results.

Still a freshman to whi world, can someone help me understand what is going on here?

The story begins after merging two tables to get one field needed from another table to create a new table.

I chose "Custom Column," and in the dialog choosing the columns from Available Columns, I wrote [Column A]-[Column B], which provided me with nothing but null values and did no subtraction at all.  Both are Whole Number Columns

[#"Total Billable Duration (Min)"]-[ISPDataBasic.Duration Min]

I also tried to pull off this daring act of complex arithmatic with the following:

"Value.Subtract([#"Total Billable Duration (Min)"],[ISPDataBasic.Duration Min])"

It did not work either.  I do not understand.  (edited ... previously said it did work.  It does not.)

One thing I did note; it changed what I expected to be simply [Total Billable Duration (Min)] into [#"Total Billable Duration (Min)"].

When attempting to correct it to remove the # and the quotes, it indicated there was Invalid Identifier (screen shot below).  Still, that is no help.  What it transformed that portion of the equation into does not work.