diff --git a/src/experimaestro/core/objects/config.py b/src/experimaestro/core/objects/config.py
index 2b495eeb..1c7f7e40 100644
--- a/src/experimaestro/core/objects/config.py
+++ b/src/experimaestro/core/objects/config.py
@@ -344,6 +344,14 @@ def seal(self, context: ConfigWalkContext):
         Arguments:
             - context: the generation context
         """
+        subconfigs = [
+            v.__xpm__
+            for v in self.values.values()
+            if isinstance(v, Config) and v.__xpm__.task is None
+        ]
+
+        if any(v._has_generated_value for v in subconfigs):
+            raise AttributeError("Cannot seal a configuration with generated values")
 
         class Sealer(ConfigWalk):
             def preprocess(self, config: ConfigMixin):
diff --git a/src/experimaestro/tests/test_generators.py b/src/experimaestro/tests/test_generators.py
index ac4d799e..7695d23d 100644
--- a/src/experimaestro/tests/test_generators.py
+++ b/src/experimaestro/tests/test_generators.py
@@ -14,7 +14,7 @@ class Learner(Task):
     x: Param[int]
 
 
-def test_generators_reuse():
+def test_generators_reuse_on_submit():
     # We have one way to select the best model
     validation = Validation.C()
 
@@ -31,3 +31,31 @@ def test_generators_reuse():
         # Here we have a problem...
         # the path is still the previous one
         Learner.C(x=2, validation=validation).submit(workspace=workspace)
+
+
+def test_generators_delayed_submit():
+    workspace = Workspace(
+        Settings(),
+        WorkspaceSettings("test_generators_simple", path=Path("/tmp")),
+        run_mode=RunMode.DRY_RUN,
+    )
+    validation = Validation.C()
+    task1 = Learner.C(x=1, validation=validation)
+    task2 = Learner.C(x=2, validation=validation)
+    task1.submit(workspace=workspace)
+    with pytest.raises((AttributeError)):
+        task2.submit(workspace=workspace)
+
+
+def test_generators_reuse_on_set():
+    workspace = Workspace(
+        Settings(),
+        WorkspaceSettings("test_generators_simple", path=Path("/tmp")),
+        run_mode=RunMode.DRY_RUN,
+    )
+    validation = Validation.C()
+    Learner.C(x=1, validation=validation).submit(workspace=workspace)
+    with pytest.raises((AttributeError)):
+        # We should not be able to *create* a second task with the same validation,
+        # even without submitting it
+        Learner.C(x=2, validation=validation)