diff --git a/server/core/service.cc b/server/core/service.cc
index b6fe71c83..105fa052b 100644
--- a/server/core/service.cc
+++ b/server/core/service.cc
@@ -1621,6 +1621,11 @@ int service_refresh_users(SERVICE *service)
     ss_dassert(self >= 0);
     time_t now = time(NULL);
 
+    if ((service->capabilities & ACAP_TYPE_ASYNC) == 0)
+    {
+        spinlock_acquire(&service->spin);
+    }
+
     /* Check if refresh rate limit has been exceeded */
     if ((now < service->rate_limits[self].last + USERS_REFRESH_TIME) ||
         (service->rate_limits[self].nloads >= USERS_REFRESH_MAX_PER_TIME))
@@ -1669,6 +1674,11 @@ int service_refresh_users(SERVICE *service)
         }
     }
 
+    if ((service->capabilities & ACAP_TYPE_ASYNC) == 0)
+    {
+        spinlock_release(&service->spin);
+    }
+
     return ret;
 }
 
diff --git a/server/modules/authenticator/MySQLAuth/mysql_auth.c b/server/modules/authenticator/MySQLAuth/mysql_auth.c
index 25aee7e13..d9b0d4614 100644
--- a/server/modules/authenticator/MySQLAuth/mysql_auth.c
+++ b/server/modules/authenticator/MySQLAuth/mysql_auth.c
@@ -98,7 +98,7 @@ MXS_MODULE* MXS_CREATE_MODULE()
         MXS_AUTHENTICATOR_VERSION,
         "The MySQL client to MaxScale authenticator implementation",
         "V1.1.0",
-        MXS_NO_MODULE_CAPABILITIES,
+        ACAP_TYPE_ASYNC,
         &MyObject,
         NULL, /* Process init. */
         NULL, /* Process finish. */