diff --git a/components/style/properties/helpers.mako.rs b/components/style/properties/helpers.mako.rs
index 0f9a7244f3c..374cd6a011e 100644
--- a/components/style/properties/helpers.mako.rs
+++ b/components/style/properties/helpers.mako.rs
@@ -75,6 +75,8 @@
                     } else {
                         % if allow_empty:
                             try!(dest.write_str("none"));
+                        % else:
+                            error!("Found empty value for property ${name}");
                         % endif
                     }
                     for i in iter {
@@ -97,6 +99,8 @@
                     } else {
                         % if allow_empty:
                             try!(dest.write_str("none"));
+                        % else:
+                            error!("Found empty value for property ${name}");
                         % endif
                     }
                     for i in iter {