diff -u shadow-4.0.3/debian/passwd.config shadow-4.0.3/debian/passwd.config
--- shadow-4.0.3/debian/passwd.config
+++ shadow-4.0.3/debian/passwd.config
@@ -291,7 +291,8 @@
 					userdefault="$(echo $RET | sed 's/ .*//' | tr A-Z a-z)"
 				    ;;
 				esac
-				if test -n "$userdefault"; then
+				if test -n "$userdefault" && \
+				   expr "$userdefault" : '[a-z][a-z0-9]*$' >/dev/null; then
 					db_set passwd/username "$userdefault"
 				fi
 			fi
diff -u shadow-4.0.3/debian/changelog shadow-4.0.3/debian/changelog
--- shadow-4.0.3/debian/changelog
+++ shadow-4.0.3/debian/changelog
@@ -1,3 +1,9 @@
+shadow (1:4.0.3-30.7ubuntu10) hoary; urgency=low
+
+  * Never generate invalid default usernames (part of Ubuntu #668).
+
+ -- Colin Watson <cjwatson@ubuntu.com>  Fri,  4 Mar 2005 11:09:13 +0000
+
 shadow (1:4.0.3-30.7ubuntu9) hoary; urgency=low
 
   * Avoid infinite loop when backing up from user password mismatch