Index: trunk/data/validator/numeric.mapcss
===================================================================
--- trunk/data/validator/numeric.mapcss	(revision 6768)
+++ trunk/data/validator/numeric.mapcss	(revision 6769)
@@ -135,2 +135,7 @@
   assertNoMatch: "way incline=10°";
 }
+
+/* see ticket #9631 */
+*[population !~ /^[0-9]+$/ ] {
+  throwWarning: tr("{0} must be a numeric value", "{0.key}");
+}