Index: lib/redmine/plugin.rb
===================================================================
--- tmp/wkAUI9_plugin.rb
+++ lib/redmine/plugin.rb
@@ -267,7 +267,12 @@ module Redmine
       arg = { :version_or_higher => arg } unless arg.is_a?(Hash)
       arg.assert_valid_keys(:version, :version_or_higher)
 
-      plugin = Plugin.find(plugin_name)
+      begin
+        plugin = Plugin.find(plugin_name)
+      rescue
+        raise PluginNotFound.new("#{id} plugin requires plugin #{plugin_name}")
+      end
+
       current = plugin.version.split('.').collect(&:to_i)
 
       arg.each do |k, v|