diff --git a/gns3server/controller/project.py b/gns3server/controller/project.py
index 38d3c4ed..c0dab9cb 100644
--- a/gns3server/controller/project.py
+++ b/gns3server/controller/project.py
@@ -289,7 +289,10 @@ class Project:
         if '{0}' in base_name or '{id}' in base_name:
             # base name is a template, replace {0} or {id} by an unique identifier
             for number in range(1, 1000000):
-                name = base_name.format(number, id=number, name="Node")
+                try:
+                    name = base_name.format(number, id=number, name="Node")
+                except KeyError as e:
+                    raise aiohttp.web.HTTPConflict(text="{" + e.args[0] + "} is not a valid replacement string in the node name")
                 if name not in self._allocated_node_names:
                     self._allocated_node_names.add(name)
                     return name