diff --git a/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/informer/DefaultPrimaryToSecondaryIndex.java b/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/informer/DefaultPrimaryToSecondaryIndex.java
index 113deef767..a1a5a96d36 100644
--- a/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/informer/DefaultPrimaryToSecondaryIndex.java
+++ b/operator-framework-core/src/main/java/io/javaoperatorsdk/operator/processing/event/source/informer/DefaultPrimaryToSecondaryIndex.java
@@ -33,9 +33,14 @@ public synchronized void onDelete(R resource) {
     primaryResources.forEach(
         primaryResource -> {
           var secondaryResources = index.get(primaryResource);
-          secondaryResources.remove(ResourceID.fromResource(resource));
-          if (secondaryResources.isEmpty()) {
-            index.remove(primaryResource);
+          // this can be null in just very special cases, like when the secondaryToPrimaryMapper is
+          // changing dynamically. Like if a list of ResourceIDs mapped dynamically extended in the
+          // mapper between the onAddOrUpdate and onDelete is called.
+          if (secondaryResources != null) {
+            secondaryResources.remove(ResourceID.fromResource(resource));
+            if (secondaryResources.isEmpty()) {
+              index.remove(primaryResource);
+            }
           }
         });
   }