diff --git a/packages/core/src/exposed-thing.ts b/packages/core/src/exposed-thing.ts
index 1b67a1560..81eca8525 100644
--- a/packages/core/src/exposed-thing.ts
+++ b/packages/core/src/exposed-thing.ts
@@ -607,7 +607,7 @@ export default class ExposedThing extends TD.Thing implements WoT.ExposedThing {
     public async handleObserveProperty(
         name: string,
         listener: ContentListener,
-        options?: WoT.InteractionOptions & { formIndex: number }
+        options: WoT.InteractionOptions & { formIndex: number }
     ): Promise<void> {
         if (this.properties[name]) {
             Helpers.validateInteractionOptions(this, this.properties[name], options);
@@ -615,7 +615,7 @@ export default class ExposedThing extends TD.Thing implements WoT.ExposedThing {
                 this.properties[name],
                 "property",
                 "observeproperty",
-                options?.formIndex
+                options.formIndex
             );
 
             if (formIndex !== -1) {
@@ -639,7 +639,7 @@ export default class ExposedThing extends TD.Thing implements WoT.ExposedThing {
     public handleUnobserveProperty(
         name: string,
         listener: ContentListener,
-        options?: WoT.InteractionOptions & { formIndex: number }
+        options: WoT.InteractionOptions & { formIndex: number }
     ): void {
         if (this.properties[name]) {
             Helpers.validateInteractionOptions(this, this.properties[name], options);
@@ -647,7 +647,7 @@ export default class ExposedThing extends TD.Thing implements WoT.ExposedThing {
                 this.properties[name],
                 "property",
                 "unobserveproperty",
-                options?.formIndex
+                options.formIndex
             );
 
             if (formIndex !== -1) {