diff --git a/src/module/rolls/check-factory.ts b/src/module/rolls/check-factory.ts
index 57e0ac5..44339b4 100644
--- a/src/module/rolls/check-factory.ts
+++ b/src/module/rolls/check-factory.ts
@@ -4,10 +4,10 @@ import { DS4 } from "../config";
  * Provides default values for all arguments the `CheckFactory` expects.
  */
 class DefaultCheckOptions implements DS4CheckFactoryOptions {
-    maxCritSuccess = 1;
-    minCritFailure = 20;
-    useSlayingDice = false;
-    rollMode: DS4RollMode = "roll";
+    readonly maxCritSuccess = 1;
+    readonly minCritFailure = 20;
+    readonly useSlayingDice = false;
+    readonly rollMode: DS4RollMode = "roll";
 
     mergeWith(other: Partial<DS4CheckFactoryOptions>): DS4CheckFactoryOptions {
         return { ...this, ...other } as DS4CheckFactoryOptions;