Look in your _settings.scss. There must be this lines:
$row-width: rem-calc(1000);
$column-gutter: rem-calc(30);
$total-columns: 12;
If you wish to modify them or just solve this problem, try to comment them out and specify $rem-base with you font size in px:
$base-font-size: 14px;
$rem-base: $base-font-size;