diff --git a/common/city.cpp b/common/city.cpp
index 96b35d7b58..0772e700e0 100644
--- a/common/city.cpp
+++ b/common/city.cpp
@@ -2247,6 +2247,10 @@ void add_specialist_output(
     // This is more than just an optimization. For governors that forbid
     // specialists, the cache may not be filled.
     if (count > 0) {
+      // If there is a cache it must not be empty.
+      fc_assert_action(!pcsoutputs || !pcsoutputs->empty(),
+                       pcsoutputs = nullptr);
+
       output_type_iterate(stat_index)
       {
         int amount = pcsoutputs
@@ -2870,6 +2874,9 @@ void set_city_production(struct city *pcity,
   trade_routes_iterate_end;
   pcity->prod[O_GOLD] += get_city_tithes_bonus(pcity);
 
+  // If there is a cache it must not be empty.
+  fc_assert_action(!pcwaste || !pcwaste->empty(), pcwaste = nullptr);
+
   /* Account for waste.  Note that waste is calculated before tax income is
    * calculated, so if you had "science waste" it would not include taxed
    * science.  However waste is calculated after the bonuses are multiplied