diff --git a/src/rougail/error.py b/src/rougail/error.py
index 178370c58..1449e23f3 100644
--- a/src/rougail/error.py
+++ b/src/rougail/error.py
@@ -73,3 +73,15 @@ class DictConsistencyError(Exception):
 
 class UpgradeError(Exception):
     """Error during XML upgrade"""
+
+## ---- generic exceptions ----
+
+class NotFoundError(Exception):
+    "not found error"
+    pass
+
+class VariableNotFoundError(NotFoundError):
+    "Variable was not found"
+    def __init__(self, errmsg, varname):
+        self.varname = varname
+
diff --git a/src/rougail/object_model.py b/src/rougail/object_model.py
index 8e3a02c9c..42816d6a6 100644
--- a/src/rougail/object_model.py
+++ b/src/rougail/object_model.py
@@ -30,7 +30,7 @@ from pydantic import (
     ConfigDict,
 )
 from .utils import get_jinja_variable_to_param, get_realpath
-
+from .error import VariableNotFoundError
 
 BASETYPE = Union[StrictBool, StrictInt, StrictFloat, StrictStr, None]
 
@@ -303,8 +303,9 @@ class VariableCalculation(Calculation):
         variable_path = self.get_realpath(self.variable)
         variable, suffix, dynamic = objectspace.paths.get_with_dynamic(variable_path)
         if not variable:
-            raise Exception(f"pffff {variable_path}")
+            raise VariableNotFoundError(f"Variable not found {variable_path}", variable_path)
         if not isinstance(variable, objectspace.variable):
+            # FIXME remove the pfff
             raise Exception("pfff it's a family")
         param = {
             "type": "variable",
diff --git a/src/rougail/tiramisureflector.py b/src/rougail/tiramisureflector.py
index 63717e318..a03de68f3 100644
--- a/src/rougail/tiramisureflector.py
+++ b/src/rougail/tiramisureflector.py
@@ -33,7 +33,7 @@ from json import dumps
 from os.path import isfile, basename
 
 from .i18n import _
-from .error import DictConsistencyError
+from .error import DictConsistencyError, VariableNotFoundError
 from .utils import normalize_family
 from .object_model import Calculation, CONVERT_OPTION
 
@@ -575,15 +575,19 @@ class Family(Common):
         children = []
         for path in self.objectspace.parents[self.elt.path]:
             children.append(self.objectspace.paths[path])
-        keys["children"] = (
-            "["
-            + ", ".join(
-                [
-                    self.tiramisu.reflector_objects[child.path].get(
-                        self.calls, self.elt.path
-                    )
-                    for child in children
-                ]
+        try:
+            keys["children"] = (
+                "["
+                + ", ".join(
+                    [
+                        self.tiramisu.reflector_objects[child.path].get(
+                            self.calls, self.elt.path
+                        )
+                        for child in children
+                    ]
+                )
+                + "]"
             )
-            + "]"
-        )
+        except VariableNotFoundError as exc:
+            msg = f"The variable '{exc.varname}' is mandatory for the dynamic family"
+            raise DictConsistencyError(msg, 88, self.elt.xmlfiles)