Incompatible units in _modal.scss

[jbgh2]

I followed the installation instructions and when I ran gulp serve I got this error message:

Details:
status: 1
file: /Stellar/node_modules/bootstrap/scss/_modal.scss
line: 177
column: 12
formatted: Error: Incompatible units: 'rem' and 'px'.
on line 177 of node_modules/bootstrap/scss/_modal.scss
from line 38 of node_modules/bootstrap/scss/bootstrap.scss
from line 121 of scss/style.scss

padding: $modal-inner-padding - $modal-footer-margin-between / 2;

-----------^

messageFormatted: node_modules\bootstrap\scss\_modal.scss

Error: Incompatible units: 'rem' and 'px'.
on line 177 of node_modules/bootstrap/scss/_modal.scss
from line 38 of node_modules/bootstrap/scss/bootstrap.scss
from line 121 of scss/style.scss

padding: $modal-inner-padding - $modal-footer-margin-between / 2;

-----------^

messageOriginal: Incompatible units: 'rem' and 'px'.
relativePath: node_modules\bootstrap\scss\_modal.scss

I'm using Windows 10.
Node: 12.14.1
NPM: 6.13.4
Gulp: CLI: 2.2.0, Local: 4.0.2