diff --git a/src/vnet/session/session_api.c b/src/vnet/session/session_api.c
index 3d70733..ff11bcb 100644
--- a/src/vnet/session/session_api.c
+++ b/src/vnet/session/session_api.c
@@ -534,7 +534,8 @@
 
   if ((rv = vnet_application_attach (a)))
     {
-      clib_warning ("attach returned: %d", rv);
+      clib_warning ("attach returned: %U", format_session_error, rv);
+      rv = VNET_API_ERROR_UNSPECIFIED;
       vec_free (a->namespace_id);
       goto done;
     }
@@ -641,7 +642,9 @@
   rv = vnet_app_worker_add_del (&args);
   if (rv)
     {
-      clib_warning ("app worker add/del returned: %d", rv);
+      clib_warning ("app worker add/del returned: %U", format_session_error,
+		    rv);
+      rv = VNET_API_ERROR_UNSPECIFIED;
       goto done;
     }
 
@@ -709,6 +712,12 @@
       a->app_index = app->app_index;
       a->api_client_index = mp->client_index;
       rv = vnet_application_detach (a);
+      if (rv)
+	{
+	  clib_warning ("vnet_application_detach: %U", format_session_error,
+			rv);
+	  rv = VNET_API_ERROR_UNSPECIFIED;
+	}
     }
 
 done:
@@ -934,7 +943,10 @@
 
   rv = vnet_session_rule_add_del (&args);
   if (rv)
-    clib_warning ("rule add del returned: %d", rv);
+    {
+      clib_warning ("rule add del returned: %U", format_session_error, rv);
+      rv = VNET_API_ERROR_UNSPECIFIED;
+    }
   vec_free (table_args->tag);
   REPLY_MACRO (VL_API_SESSION_RULE_ADD_DEL_REPLY);
 }
@@ -1149,6 +1161,12 @@
     }
   ckpair_index = clib_net_to_host_u32 (mp->index);
   rv = vnet_app_del_cert_key_pair (ckpair_index);
+  if (rv)
+    {
+      clib_warning ("vnet_app_del_cert_key_pair: %U", format_session_error,
+		    rv);
+      rv = VNET_API_ERROR_UNSPECIFIED;
+    }
 
 done:
   REPLY_MACRO (VL_API_APP_DEL_CERT_KEY_PAIR_REPLY);
@@ -1367,7 +1385,7 @@
   app = application_get_if_valid (mp->app_index);
   if (!app)
     {
-      rv = VNET_API_ERROR_INVALID_VALUE;
+      rv = SESSION_E_INVALID;
       goto done;
     }
 
@@ -1382,7 +1400,8 @@
   rv = vnet_app_worker_add_del (&args);
   if (rv)
     {
-      clib_warning ("app worker add/del returned: %d", rv);
+      clib_warning ("app worker add/del returned: %U", format_session_error,
+		    rv);
       goto done;
     }
 
@@ -1730,7 +1749,7 @@
       if (err)
 	{
 	  clib_error_report (err);
-	  return VNET_API_ERROR_SYSCALL_ERROR_1;
+	  return SESSION_E_SYSCALL;
 	}
     }