diff --git a/src/main/java/eu/dnetlib/uoaauthorizationlibrary/security/AuthorizationService.java b/src/main/java/eu/dnetlib/uoaauthorizationlibrary/security/AuthorizationService.java
index 41664b2..42bdda9 100644
--- a/src/main/java/eu/dnetlib/uoaauthorizationlibrary/security/AuthorizationService.java
+++ b/src/main/java/eu/dnetlib/uoaauthorizationlibrary/security/AuthorizationService.java
@@ -70,7 +70,7 @@ public class AuthorizationService {
 
     public String getAaiId() {
         Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
-        if (authentication != null) {
+        if (authentication != null && (authentication.getPrincipal() instanceof User)) {
             User user = (User) authentication.getPrincipal();
             return user.getPassword();
         }