diff --git a/openfisca_core/simulations/_build_default_simulation.py b/openfisca_core/simulations/_build_default_simulation.py
index 504d701ae..f8828ff59 100644
--- a/openfisca_core/simulations/_build_default_simulation.py
+++ b/openfisca_core/simulations/_build_default_simulation.py
@@ -5,9 +5,9 @@
 import numpy
 
 from .simulation import Simulation
-from .types import CoreEntity, CorePopulation, TaxBenefitSystem
+from .types import CoreEntity, GroupPopulation, TaxBenefitSystem
 
-Populations: TypeAlias = dict[str, CorePopulation[CoreEntity]]
+Populations: TypeAlias = dict[str, GroupPopulation[CoreEntity]]
 
 
 class _BuildDefaultSimulation:
diff --git a/openfisca_core/simulations/_build_from_variables.py b/openfisca_core/simulations/_build_from_variables.py
index 8858cd6b8..4721ebca6 100644
--- a/openfisca_core/simulations/_build_from_variables.py
+++ b/openfisca_core/simulations/_build_from_variables.py
@@ -10,9 +10,9 @@
 from ._build_default_simulation import _BuildDefaultSimulation
 from ._guards import is_variable_dated
 from .simulation import Simulation
-from .types import CoreEntity, CorePopulation, TaxBenefitSystem, Variables
+from .types import CoreEntity, GroupPopulation, TaxBenefitSystem, Variables
 
-Populations: TypeAlias = dict[str, CorePopulation[CoreEntity]]
+Populations: TypeAlias = dict[str, GroupPopulation[CoreEntity]]
 
 
 class _BuildFromVariables:
diff --git a/openfisca_core/simulations/types.py b/openfisca_core/simulations/types.py
index 5a8e23fd3..b57345039 100644
--- a/openfisca_core/simulations/types.py
+++ b/openfisca_core/simulations/types.py
@@ -112,9 +112,7 @@ class Period(t.Period, Protocol):
 
 
 class CorePopulation(t.CorePopulation, Protocol[D]):
-    count: int
     entity: D
-    ids: t.Array[String]
 
     def get_holder(self, __variable_name: str) -> Holder[V]:
         ...
@@ -213,7 +211,7 @@ def get_variable(
 
     def instantiate_entities(
         self,
-    ) -> dict[str, SinglePopulation[E]]:
+    ) -> dict[str, GroupPopulation[E]]:
         ...
 
 
diff --git a/openfisca_core/types.py b/openfisca_core/types.py
index 168a2ed29..95f3109b5 100644
--- a/openfisca_core/types.py
+++ b/openfisca_core/types.py
@@ -125,7 +125,9 @@ def unit(self) -> Any:
 
 
 class CorePopulation(Protocol):
+    count: int
     entity: Any
+    ids: Array[numpy.str_]
 
     @abc.abstractmethod
     def get_holder(self, variable_name: Any) -> Any: