diff --git a/src/js/game/components/underground_belt.js b/src/js/game/components/underground_belt.js
index b9aa3922..c54c5a93 100644
--- a/src/js/game/components/underground_belt.js
+++ b/src/js/game/components/underground_belt.js
@@ -18,7 +18,7 @@ export class UndergroundBeltComponent extends Component {
     static getSchema() {
         return {
             mode: types.enum(enumUndergroundBeltMode),
-            pendingItems: types.array(types.pair(types.obj(gItemRegistry), types.number)),
+            pendingItems: types.array(types.pair(types.obj(gItemRegistry), types.ufloat)),
         };
     }
 
diff --git a/src/js/savegame/serialization_data_types.js b/src/js/savegame/serialization_data_types.js
index 5020e09f..332dc274 100644
--- a/src/js/savegame/serialization_data_types.js
+++ b/src/js/savegame/serialization_data_types.js
@@ -1079,6 +1079,8 @@ export class TypePair extends BaseDataType {
      */
     constructor(type1, type2) {
         super();
+        assert(type1 && type1 instanceof BaseDataType, "bad first type given for pair");
+        assert(type2 && type2 instanceof BaseDataType, "bad second type given for pair");
         this.type1 = type1;
         this.type2 = type2;
     }