Don't generate an IN condition with an empty list.

app/controllers/trace_controller.rb

@@ -57,8 +57,11 @@ class TraceController < ApplicationController
       @tag = params[:tag]
 
       files = Tracetag.find_all_by_tag(params[:tag]).collect { |tt| tt.gpx_id }
+
+      if files.length > 0
         conditions[0] += " AND gpx_files.id IN (#{files.join(',')})"
       end
+    end
     
     conditions[0] += " AND gpx_files.visible = ?"
     conditions << true