diff --git a/ext/js/templates/template-renderer.js b/ext/js/templates/template-renderer.js
index e27ad03c..a06298aa 100644
--- a/ext/js/templates/template-renderer.js
+++ b/ext/js/templates/template-renderer.js
@@ -29,6 +29,8 @@ class TemplateRenderer {
         this._stateStack = null;
         this._dataTypes = new Map();
         this._requirements = null;
+        this._cleanupCallbacks = null;
+        this._customData = null;
     }
 
     registerDataType(name, {modifier=null, composeData=null}) {
@@ -85,15 +87,21 @@ class TemplateRenderer {
     }
 
     _renderTemplate(instance, data) {
+        const cleanupCallbacks = [];
+        const requirements = [];
         try {
-            const requirements = [];
+            this._customData = {};
             this._stateStack = [new Map()];
             this._requirements = requirements;
+            this._cleanupCallbacks = cleanupCallbacks;
             const result = instance(data).trim();
             return {result, requirements};
         } finally {
+            for (const callback of cleanupCallbacks) { callback(); }
             this._stateStack = null;
             this._requirements = null;
+            this._cleanupCallbacks = null;
+            this._customData = null;
         }
     }