diff --git a/src/main/java/org/olat/modules/webFeed/ui/FeedMainController.java b/src/main/java/org/olat/modules/webFeed/ui/FeedMainController.java
index d4f77ba0a9e90f94d74e85133090bc9fa46f8eb8..b3f2d231afa8bc08405da90ef58969cbf50d4897 100644
--- a/src/main/java/org/olat/modules/webFeed/ui/FeedMainController.java
+++ b/src/main/java/org/olat/modules/webFeed/ui/FeedMainController.java
@@ -117,6 +117,9 @@ public class FeedMainController extends BasicController implements Activateable2
 			putInitialPanel(vcMain);
 		} else {
 			String authorFullname = userManager.getUserDisplayName(feed.getAuthor());
+			if(authorFullname == null) {
+				authorFullname = "???";
+			}
 			helper = new FeedViewHelper(feed, getIdentity(), authorFullname, uiFactory.getTranslator(), courseId, nodeId, callback);
 			CoordinatorManager.getInstance().getCoordinator().getEventBus().registerFor(this, ureq.getIdentity(), feed);
 			display(ureq, wControl, displayConfig);
diff --git a/src/main/java/org/olat/user/UserManagerImpl.java b/src/main/java/org/olat/user/UserManagerImpl.java
index 796c793bc9dedbe40c58eeee23fdd911d1b1f1e4..78294a8b2162364e6376ab6fd6ab0e83be4b093b 100644
--- a/src/main/java/org/olat/user/UserManagerImpl.java
+++ b/src/main/java/org/olat/user/UserManagerImpl.java
@@ -433,6 +433,10 @@ public class UserManagerImpl extends UserManager {
 
 	@Override
 	public String getUsername(Long identityKey) {
+		if(identityKey == null || identityKey.longValue() <= 0) {
+			return null;
+		}
+		
 		String username = userToNameCache.get(identityKey);
 		if(username == null) {
 			IdentityShort identity = securityManager.loadIdentityShortByKey(identityKey);
@@ -444,6 +448,7 @@ public class UserManagerImpl extends UserManager {
 
 	@Override
 	public String getUserDisplayName(String username) {
+		if(username == null) return null;
 		String fullName = userToFullnameCache.get(username);
 		if(fullName == null) {
 			List<IdentityShort> identities = securityManager.findShortIdentitiesByName(Collections.singletonList(username));