diff --git a/servo/components/selectors/parser.rs b/servo/components/selectors/parser.rs
index 56f801d06133..865424120e84 100644
--- a/servo/components/selectors/parser.rs
+++ b/servo/components/selectors/parser.rs
@@ -1503,6 +1503,14 @@ impl<Impl: SelectorImpl> ToCss for Component<Impl> {
             AttributeOther(ref attr_selector) => attr_selector.to_css(dest),
 
             // Pseudo-classes
+            Negation(ref arg) => {
+                dest.write_str(":not(")?;
+                for component in arg.iter() {
+                    component.to_css(dest)?;
+                }
+                dest.write_str(")")
+            },
+
             FirstChild => dest.write_str(":first-child"),
             LastChild => dest.write_str(":last-child"),
             OnlyChild => dest.write_str(":only-child"),
@@ -1532,11 +1540,10 @@ impl<Impl: SelectorImpl> ToCss for Component<Impl> {
                 write_affine(dest, a, b)?;
                 dest.write_char(')')
             },
-            Is(ref list) | Where(ref list) | Negation(ref list) => {
+            Is(ref list) | Where(ref list) => {
                 match *self {
                     Where(..) => dest.write_str(":where(")?,
                     Is(..) => dest.write_str(":is(")?,
-                    Negation(..) => dest.write_str(":not(")?,
                     _ => unreachable!(),
                 }
                 serialize_selector_list(list.iter(), dest)?;