diff --git a/lib/ansible/modules/files/xml.py b/lib/ansible/modules/files/xml.py
index df1a53947f..389a87b052 100644
--- a/lib/ansible/modules/files/xml.py
+++ b/lib/ansible/modules/files/xml.py
@@ -545,8 +545,11 @@ def set_target_inner(module, tree, xpath, namespaces, attribute, value):
         if not is_node(tree, xpath, namespaces):
             changed = check_or_make_target(module, tree, xpath, namespaces)
     except Exception as e:
-        module.fail_json(msg="Xpath %s causes a failure: %s\n  -- tree is %s" %
-                             (xpath, e, etree.tostring(tree, pretty_print=True)), exception=traceback.format_exc(e))
+        missing_namespace = ""
+        if len(tree.getroot().nsmap) > 0 and ":" not in xpath:
+            missing_namespace = "XML document has namespace(s) defined, but no namespace prefix(es) used in xpath!\n"
+        module.fail_json(msg="%sXpath %s causes a failure: %s\n  -- tree is %s" %
+                             (missing_namespace, xpath, e, etree.tostring(tree, pretty_print=True)), exception=traceback.format_exc())
 
     if not is_node(tree, xpath, namespaces):
         module.fail_json(msg="Xpath %s does not reference a node! tree is %s" %