diff --git a/spring-ldap/src/main/java/org/springframework/ldap/authentication/AcegiAuthenticationSource.java b/spring-ldap/src/main/java/org/springframework/ldap/authentication/AcegiAuthenticationSource.java
index e7153f2b..2a8fa91e 100644
--- a/spring-ldap/src/main/java/org/springframework/ldap/authentication/AcegiAuthenticationSource.java
+++ b/spring-ldap/src/main/java/org/springframework/ldap/authentication/AcegiAuthenticationSource.java
@@ -18,12 +18,12 @@ package org.springframework.ldap.authentication;
 
 import org.acegisecurity.Authentication;
 import org.acegisecurity.context.SecurityContextHolder;
+import org.acegisecurity.providers.anonymous.AnonymousAuthenticationToken;
 import org.acegisecurity.userdetails.ldap.LdapUserDetails;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.springframework.ldap.core.AuthenticationSource;
 
-
 /**
  * An AuthenticationSource to retrieve authentication information stored in
  * Acegi's SecurityContextHolder. Use Acegi's LdapAuthenticationProvider have a
@@ -47,13 +47,19 @@ public class AcegiAuthenticationSource implements AuthenticationSource {
                 .getAuthentication();
         if (authentication != null) {
             Object principal = authentication.getPrincipal();
-            if (!(principal instanceof LdapUserDetails)) {
+            if (principal instanceof LdapUserDetails) {
+                LdapUserDetails details = (LdapUserDetails) principal;
+                return details.getDn();
+            } else if (authentication instanceof AnonymousAuthenticationToken) {
+                if (log.isDebugEnabled()) {
+                    log
+                            .debug("Anonymous Authentication, returning empty String as Principal");
+                }
+                return "";
+            } else {
                 throw new IllegalArgumentException(
                         "The principal property of the authentication object -"
                                 + "needs to be a LdapUserDetails.");
-            } else {
-                LdapUserDetails details = (LdapUserDetails) principal;
-                return details.getDn();
             }
         } else {
             log.warn("No Authentication object set in SecurityContext - "
diff --git a/spring-ldap/src/test/java/org/springframework/ldap/authentication/AcegiAuthenticationSourceTest.java b/spring-ldap/src/test/java/org/springframework/ldap/authentication/AcegiAuthenticationSourceTest.java
index 36c46a9b..dd9d89d4 100644
--- a/spring-ldap/src/test/java/org/springframework/ldap/authentication/AcegiAuthenticationSourceTest.java
+++ b/spring-ldap/src/test/java/org/springframework/ldap/authentication/AcegiAuthenticationSourceTest.java
@@ -18,10 +18,10 @@ package org.springframework.ldap.authentication;
 
 import junit.framework.TestCase;
 
-
 import org.acegisecurity.Authentication;
 import org.acegisecurity.GrantedAuthority;
 import org.acegisecurity.context.SecurityContextHolder;
+import org.acegisecurity.providers.anonymous.AnonymousAuthenticationToken;
 import org.acegisecurity.userdetails.User;
 import org.acegisecurity.userdetails.ldap.LdapUserDetails;
 import org.easymock.MockControl;
@@ -83,7 +83,7 @@ public class AcegiAuthenticationSourceTest extends TestCase {
 
         ldapUserDetailsControl.expectAndDefaultReturn(ldapUserDetailsMock
                 .getDn(), "cn=Manager");
-        
+
         SecurityContextHolder.getContext()
                 .setAuthentication(authenticationMock);
 
@@ -126,4 +126,19 @@ public class AcegiAuthenticationSourceTest extends TestCase {
         }
         verify();
     }
+
+    public void testGetPrincipalWithAnonymousAuthenticationToken() {
+
+        SecurityContextHolder.getContext().setAuthentication(
+                new AnonymousAuthenticationToken("dummy", "dummy",
+                        new GrantedAuthority[] { new DummyAuthoroty() }));
+
+        assertEquals("", tested.getPrincipal());
+    }
+
+    private static class DummyAuthoroty implements GrantedAuthority {
+        public String getAuthority() {
+            return null;
+        }
+    }
 }