diff --git a/lib/less/functions.js b/lib/less/functions.js
index 68d93a30..c6ab2340 100644
--- a/lib/less/functions.js
+++ b/lib/less/functions.js
@@ -281,6 +281,9 @@ tree.functions = {
             values[unit] = values[""] !== undefined && unit !== "" && unit === unitStatic ? values[""] : values[unit];
             j = values[unit];
             if (j === undefined) {
+                if(unitStatic !== undefined && unit !== unitStatic) {
+                    throw{ type: "Argument", message: "incompatible types" };
+                }
                 values[unit] = order.length;
                 order.push(current);
                 continue;