diff --git a/watcher/common/exception.py b/watcher/common/exception.py
index f962b3bbe..dcb489ec9 100644
--- a/watcher/common/exception.py
+++ b/watcher/common/exception.py
@@ -447,6 +447,11 @@ class InstanceNotFound(ComputeResourceNotFound):
     msg_fmt = _("The instance '%(name)s' could not be found")
 
 
+class InstanceNotMapped(ComputeResourceNotFound):
+    msg_fmt = _("The mapped compute node for instance '%(uuid)s' "
+                "could not be found.")
+
+
 class ComputeNodeNotFound(ComputeResourceNotFound):
     msg_fmt = _("The compute node %(name)s could not be found")
 
diff --git a/watcher/decision_engine/model/model_root.py b/watcher/decision_engine/model/model_root.py
index ad85bdd9c..1845638de 100644
--- a/watcher/decision_engine/model/model_root.py
+++ b/watcher/decision_engine/model/model_root.py
@@ -170,7 +170,7 @@ class ModelRoot(nx.DiGraph, base.Model):
             node = self._get_by_uuid(node_uuid)
             if isinstance(node, element.ComputeNode):
                 return node
-        raise exception.ComputeNodeNotFound(name=instance_uuid)
+        raise exception.InstanceNotMapped(uuid=instance_uuid)
 
     @lockutils.synchronized("model_root")
     def get_all_instances(self):
@@ -211,7 +211,7 @@ class ModelRoot(nx.DiGraph, base.Model):
                                key=lambda inst: inst.uuid):
             try:
                 self.get_node_by_instance_uuid(instance.uuid)
-            except (exception.InstanceNotFound, exception.ComputeNodeNotFound):
+            except exception.ComputeResourceNotFound:
                 root.append(instance.as_xml_element())
 
         return etree.tostring(root, pretty_print=True).decode('utf-8')
diff --git a/watcher/decision_engine/model/notification/nova.py b/watcher/decision_engine/model/notification/nova.py
index a2254275e..5c885bd14 100644
--- a/watcher/decision_engine/model/notification/nova.py
+++ b/watcher/decision_engine/model/notification/nova.py
@@ -160,7 +160,7 @@ class NovaNotification(base.NotificationEndpoint):
                 current_node = (
                     self.cluster_data_model.get_node_by_instance_uuid(
                         instance.uuid))
-            except exception.ComputeNodeNotFound as exc:
+            except exception.ComputeResourceNotFound as exc:
                 LOG.exception(exc)
                 # If we can't create the node,
                 # we consider the instance as unmapped