Fix luminance with non-integer Sass color channels

scss/_functions.scss

@@ -188,7 +188,7 @@ $_luminance-list: .0008 .001 .0011 .0013 .0015 .0017 .002 .0022 .0025 .0027 .003
   );
 
   @each $name, $value in $rgb {
-    $value: if(divide($value, 255) < .04045, divide(divide($value, 255), 12.92), nth($_luminance-list, $value + 1));
+    $value: if(divide($value, 255) < .04045, divide(divide($value, 255), 12.92), nth($_luminance-list, min(255, max(0, round($value))) + 1));
     $rgb: map-merge($rgb, ($name: $value));
   }
 

scss/tests/functions/_luminance.test.scss

@@ -0,0 +1,22 @@
+@import "../../functions";
+
+// Simulate Sass color channel functions returning non-integer channel values.
+@function red($color) {
+  @return 241.5;
+}
+
+@function green($color) {
+  @return 135.5;
+}
+
+@function blue($color) {
+  @return 8.5;
+}
+
+@include describe("luminance function") {
+  @include it("should round non-integer channel values before reading the luminance table") {
+    $result: luminance(#f7941c);
+
+    @include assert-true($result > 0 and $result < 1, "Non-integer color channels should not crash luminance()");
+  }
+}