diff --git a/scss/_variables.scss b/scss/_variables.scss
index d129f9fee4..8a3df44be9 100644
--- a/scss/_variables.scss
+++ b/scss/_variables.scss
@@ -21,7 +21,11 @@
   $prev-key: null;
   $prev-num: null;
   @each $key, $num in $map {
-    @if $prev-num != null and $prev-num >= $num {
+    @if $prev-num == null {
+      // Do nothing
+    } @else if not comparable($prev-num, $num) {
+      @warn "Potentially invalid value for #{$map-name}: This map must be in ascending order, but key '#{$key}' has value #{$num} whose unit makes it incomparable to #{$prev-num}, the value of the previous key '#{$prev-key}' !";
+    } @else if $prev-num >= $num {
       @warn "Invalid value for #{$map-name}: This map must be in ascending order, but key '#{$key}' has value #{$num} which isn't greater than #{$prev-num}, the value of the previous key '#{$prev-key}' !";
     }
     $prev-key: $key;