diff --git a/src/core/annotation.js b/src/core/annotation.js
index 6f6c533e1..710551639 100644
--- a/src/core/annotation.js
+++ b/src/core/annotation.js
@@ -1532,6 +1532,8 @@ class WidgetAnnotation extends Annotation {
       }
     }
 
+    assert(typeof value === "string", "Expected `value` to be a string.");
+
     value = value.trim();
 
     if (value === "") {
diff --git a/src/scripting_api/event.js b/src/scripting_api/event.js
index c57f3ddae..f4036bb26 100644
--- a/src/scripting_api/event.js
+++ b/src/scripting_api/event.js
@@ -191,7 +191,7 @@ class EventDispatcher {
       let formattedValue = null;
 
       if (this.runActions(source, source, event, "Format")) {
-        formattedValue = event.value;
+        formattedValue = event.value?.toString?.();
       }
 
       source.obj._send({
@@ -296,7 +296,7 @@ class EventDispatcher {
       savedValue = event.value = target.obj.value;
       let formattedValue = null;
       if (this.runActions(target, target, event, "Format")) {
-        formattedValue = event.value;
+        formattedValue = event.value?.toString?.();
       }
 
       target.obj._send({