Sass Variable in CSS calc() function

I'm trying to use the calc() function in a Sass stylesheet, but I'm having some issues. Here's my code:

$body_padding: 50px
body

padding-top: $body_padding

height: calc(100% - $body_padding)

If I use the literal 50px instead of my body_padding variable, I get exactly what I want. However, when I switch to the variable, this is the output:

body {

padding-top: 50px;

height: calc(100% - $body_padding); }

How can I get Sass to recognize that it needs to replace the variable within the calc function?

#sass