diff --git a/src/main/java/hudson/plugins/im/IMConnectionProvider.java b/src/main/java/hudson/plugins/im/IMConnectionProvider.java
index 1be1934..81b914c 100644
--- a/src/main/java/hudson/plugins/im/IMConnectionProvider.java
+++ b/src/main/java/hudson/plugins/im/IMConnectionProvider.java
@@ -8,7 +8,10 @@
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
+import jenkins.model.Jenkins;
 import org.acegisecurity.Authentication;
+import org.acegisecurity.userdetails.UsernameNotFoundException;
+//import org.springframework.security.core.userdetails.UsernameNotFoundException;
 
 /**
  * Abstract implementation of a provider of {@link IMConnection}s.
@@ -132,9 +135,20 @@ public Authentication getAuthentication() {
                     return null;
                 }
 
-                User u = User.get(descriptor.getHudsonUserName());
+                if (descriptor.getHudsonUserName().isBlank() && !(Jenkins.getInstance().isUseSecurity())) {
+                    return null;
+                }
 
-                return u.impersonate();
+                try {
+                    User u = User.get(descriptor.getHudsonUserName());
+                    return u.impersonate();
+                } catch (UsernameNotFoundException ue) {
+                    if (descriptor.getHudsonUserName().isBlank()) {
+                        throw new UsernameNotFoundException(
+                            "No local Jenkins user name is configured for instant messaging to act as");
+                    }
+                    throw ue;
+                }
             }
         };
     }
diff --git a/src/main/java/hudson/plugins/im/bot/Bot.java b/src/main/java/hudson/plugins/im/bot/Bot.java
index d51be02..ff8c2e6 100644
--- a/src/main/java/hudson/plugins/im/bot/Bot.java
+++ b/src/main/java/hudson/plugins/im/bot/Bot.java
@@ -26,6 +26,7 @@
 
 import jenkins.model.Jenkins;
 import jenkins.security.NotReallyRoleSensitiveCallable;
+import org.acegisecurity.userdetails.UsernameNotFoundException;
 
 /**
  * Instant messaging bot.
@@ -162,15 +163,20 @@ public void onMessage(final IMMessage msg) {
                     final BotCommand command = this.cmdsAndAliases.get(cmd);
                     if (command != null) {
                         if (isAuthenticationNeeded()) {
-                            ACL.impersonate(this.authentication.getAuthentication(), new NotReallyRoleSensitiveCallable<Void, IMException>() {
-                                private static final long serialVersionUID = 1L;
-
-                                @Override
-                                public Void call() throws IMException {
-                                    command.executeCommand(Bot.this, chat, msg, s, args);
-                                    return null;
-                                }
-                            });
+                            try {
+                                ACL.impersonate(this.authentication.getAuthentication(), new NotReallyRoleSensitiveCallable<Void, IMException>() {
+                                    private static final long serialVersionUID = 1L;
+
+                                    @Override
+                                    public Void call() throws IMException {
+                                        command.executeCommand(Bot.this, chat, msg, s, args);
+                                        return null;
+                                    }
+                                });
+                            } catch (UsernameNotFoundException ue) {
+                                this.chat.sendMessage(s.getNickname() + " This bot is not authorized to execute commands at this time, sorry!");
+                                throw ue;
+                            }
                         } else {
                             command.executeCommand(Bot.this, chat, msg, s, args);
                         }