diff --git a/extensions/guacamole-auth-sso/modules/guacamole-auth-sso-ssl/src/main/java/org/apache/guacamole/auth/ssl/SSLClientAuthenticationResource.java b/extensions/guacamole-auth-sso/modules/guacamole-auth-sso-ssl/src/main/java/org/apache/guacamole/auth/ssl/SSLClientAuthenticationResource.java
index fd3230b7d..443795001 100644
--- a/extensions/guacamole-auth-sso/modules/guacamole-auth-sso-ssl/src/main/java/org/apache/guacamole/auth/ssl/SSLClientAuthenticationResource.java
+++ b/extensions/guacamole-auth-sso/modules/guacamole-auth-sso-ssl/src/main/java/org/apache/guacamole/auth/ssl/SSLClientAuthenticationResource.java
@@ -133,7 +133,7 @@ public class SSLClientAuthenticationResource extends SSOResource {
     private String getHeader(HttpHeaders headers, String name) {
 
         List<String> values = headers.getRequestHeader(name);
-        if (values.isEmpty())
+        if (values == null || values.isEmpty())
             return null;
 
         return values.get(0);