diff --git a/src/options/sort-order.js b/src/options/sort-order.js
index 27a2198a..482e0259 100644
--- a/src/options/sort-order.js
+++ b/src/options/sort-order.js
@@ -391,14 +391,19 @@ module.exports = {
     if (a[2] !== b[2]) {
       // If unprefixed parts are different (i.e. `border` and
       // `color`), compare them:
-      return a[2] <= b[2] ? -1 : 1;
-    } else {
-      // If unprefixed parts are identical (i.e. `border` in
-      // `-moz-border` and `-o-border`), compare prefixes.
-      // They should go in the same order they are set
-      // in `prefixes` array.
-      return prefixes.indexOf(a[1]) <= prefixes.indexOf(b[1]) ? -1 : 1;
+      return a[2] < b[2] ? -1 : 1;
+    }
+
+    // If unprefixed parts are identical (i.e. `border` in
+    // `-moz-border` and `-o-border`), compare prefixes.
+    // They should be untouched if they are equal:
+    if (prefixes.indexOf(a[1]) === prefixes.indexOf(b[1])) {
+      return 0;
     }
+
+    // They should go in the same order they are set
+    // in `prefixes` array.
+    return prefixes.indexOf(a[1]) < prefixes.indexOf(b[1]) ? -1 : 1;
   },
 
   _sortNodes(nodes) {