diff --git a/cats/cats-core/src/main/java/com/netflix/spinnaker/cats/provider/DefaultProviderCache.java b/cats/cats-core/src/main/java/com/netflix/spinnaker/cats/provider/DefaultProviderCache.java
index 729fe52eeae..55d636b45c7 100644
--- a/cats/cats-core/src/main/java/com/netflix/spinnaker/cats/provider/DefaultProviderCache.java
+++ b/cats/cats-core/src/main/java/com/netflix/spinnaker/cats/provider/DefaultProviderCache.java
@@ -135,7 +135,7 @@ public void putCacheResult(String sourceAgentType, Collection<String> authoritat
         for (String type : allTypes) {
             final Collection<String> previousSet;
             if (authoritativeTypes.contains(type)) {
-                previousSet = getExistingSourceIdentifiers(type, sourceAgentType);
+                previousSet = new HashSet<>(getExistingSourceIdentifiers(type, sourceAgentType));
             } else {
                 previousSet = new HashSet<>();
             }