--- a/servo/components/style/values/computed/length.rs
+++ b/servo/components/style/values/computed/length.rs
@@ -857,16 +857,20 @@ impl ToComputedValue for specified::MozL
 
     #[inline]
     fn to_computed_value(&self, context: &Context) -> MozLength {
         match *self {
             specified::MozLength::LengthOrPercentageOrAuto(ref lopoa) => {
                 MozLength::LengthOrPercentageOrAuto(lopoa.to_computed_value(context))
             }
             specified::MozLength::ExtremumLength(ref ext) => {
+                debug_assert!(context.for_non_inherited_property.is_some(),
+                              "should check whether we're a non-inherited property");
+                context.rule_cache_conditions.borrow_mut()
+                    .set_writing_mode_dependency(context.builder.writing_mode);
                 MozLength::ExtremumLength(ext.clone())
             }
         }
     }
 
     #[inline]
     fn from_computed_value(computed: &MozLength) -> Self {
         match *computed {