diff --git a/packages/conform-dom/formdata.ts b/packages/conform-dom/formdata.ts
index 30aa5111..9c132265 100644
--- a/packages/conform-dom/formdata.ts
+++ b/packages/conform-dom/formdata.ts
@@ -128,11 +128,37 @@ export function setValue(
 						: {}
 				: valueFn(pointer[key]);
 
-		pointer[key] = newValue;
+		if (canAddProperty(pointer, key)) {
+			pointer[key] = newValue;
+		}
 		pointer = pointer[key];
 	}
 }
 
+function canAddProperty(
+	obj: Object | Array<unknown>,
+	property: string | number,
+): boolean {
+	const propertyStr =
+		typeof property === 'number' ? property.toString() : property;
+	if (Object.prototype.hasOwnProperty.call(obj, propertyStr)) {
+		const descriptor = Object.getOwnPropertyDescriptor(obj, propertyStr);
+		if (descriptor && !descriptor.writable) {
+			return false;
+		}
+	}
+	if (!Object.isExtensible(obj)) {
+		return false;
+	}
+	if (Object.isSealed(obj) && !(propertyStr in obj)) {
+		return false;
+	}
+	if (Object.isFrozen(obj) && !(propertyStr in obj)) {
+		return false;
+	}
+	return true;
+}
+
 /**
  * Retrive the value from a target object by following the paths
  */