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 55d636b45c7..28080d9f3b3 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
@@ -133,9 +133,9 @@ public void putCacheResult(String sourceAgentType, Collection<String> authoritat
         Map<String, Collection<String>> evictions = new HashMap<>();
 
         for (String type : allTypes) {
-            final Collection<String> previousSet;
+            final Set<String> previousSet;
             if (authoritativeTypes.contains(type)) {
-                previousSet = new HashSet<>(getExistingSourceIdentifiers(type, sourceAgentType));
+                previousSet = getExistingSourceIdentifiers(type, sourceAgentType);
             } else {
                 previousSet = new HashSet<>();
             }
@@ -193,7 +193,7 @@ private Collection<CacheData> buildResponse(Collection<CacheData> source) {
         return Collections.unmodifiableCollection(response);
     }
 
-    private Collection<String> getExistingSourceIdentifiers(String type, String sourceAgentType) {
+    private Set<String> getExistingSourceIdentifiers(String type, String sourceAgentType) {
         CacheData all = backingStore.get(type, ALL_ID);
         if (all == null) {
             return new HashSet<>();
@@ -202,7 +202,7 @@ private Collection<String> getExistingSourceIdentifiers(String type, String sour
         if (relationship == null) {
             return new HashSet<>();
         }
-        return relationship;
+        return relationship instanceof Set ? (Set<String>) relationship : new HashSet<>(relationship);
     }
 
     private void cacheDataType(String type, String sourceAgentType, Collection<CacheData> items) {
diff --git a/cats/cats-redis/src/main/java/com/netflix/spinnaker/cats/redis/cache/AbstractRedisCache.java b/cats/cats-redis/src/main/java/com/netflix/spinnaker/cats/redis/cache/AbstractRedisCache.java
index 47c5e64aa46..13db2e26235 100644
--- a/cats/cats-redis/src/main/java/com/netflix/spinnaker/cats/redis/cache/AbstractRedisCache.java
+++ b/cats/cats-redis/src/main/java/com/netflix/spinnaker/cats/redis/cache/AbstractRedisCache.java
@@ -37,7 +37,7 @@ public abstract class AbstractRedisCache implements WriteableCache {
 
   protected static final TypeReference<Map<String, Object>> ATTRIBUTES = new TypeReference<Map<String, Object>>() {
   };
-  protected static final TypeReference<List<String>> RELATIONSHIPS = new TypeReference<List<String>>() {
+  protected static final TypeReference<Set<String>> RELATIONSHIPS = new TypeReference<Set<String>>() {
   };
 
   private final Logger log = LoggerFactory.getLogger(getClass());