diff --git a/mocks/pvx.lua b/mocks/pvx.lua
index 34bc73c..01b1706 100644
--- a/mocks/pvx.lua
+++ b/mocks/pvx.lua
@@ -54,6 +54,14 @@ local pvxMock = {
             end
         end
 
+        function t.xavp_is_null(xavp_name)
+            local private_id = "xavp:" .. xavp_name
+            if not t.pv.vars[private_id] then
+                return 1
+            end
+            return -1
+        end
+
         function t.xavp_get(xavp_name)
             return t._get_xavp(xavp_name, "NULL_NONE")
         end
diff --git a/tests/mocks_pvx.lua b/tests/mocks_pvx.lua
index 34efa53..5559754 100644
--- a/tests/mocks_pvx.lua
+++ b/tests/mocks_pvx.lua
@@ -49,6 +49,11 @@ TestPVXMock = {}
         self.pv.vars = {}
     end
 
+    function TestPVXMock:test_xavp_is_null()
+        lu.assertEquals(self.pvx.xavp_is_null("test"), -1)
+        lu.assertEquals(self.pvx.xavp_is_null("whatever"), 1)
+    end
+
     function TestPVXMock:test_xavp_get()
         local l = self.pvx.xavp_get("test")
         local m = tostring(self.pv.vars["xavp:test"])