diff --git a/src/everest/config/everest_config.py b/src/everest/config/everest_config.py
index ce22b353649..1e464652473 100644
--- a/src/everest/config/everest_config.py
+++ b/src/everest/config/everest_config.py
@@ -114,8 +114,7 @@ class EverestConfig(BaseModelWithPropertySupport):  # type: ignore
         default=OptimizationConfig(),
         description="Optimizer options",
     )
-    model: ModelConfig | None = Field(
-        default=ModelConfig(),
+    model: ModelConfig = Field(
         description="Configuration of the Everest model",
     )
 
diff --git a/src/everest/config/model_config.py b/src/everest/config/model_config.py
index 9fe95affe02..84b67594e7f 100644
--- a/src/everest/config/model_config.py
+++ b/src/everest/config/model_config.py
@@ -5,10 +5,10 @@
 
 class ModelConfig(BaseModel, extra="forbid"):  # type: ignore
     realizations: list[NonNegativeInt] = Field(
-        default_factory=lambda: [],
         description="""List of realizations to use in optimization ensemble.
 
 Typically, this is a list [0, 1, ..., n-1] of all realizations in the ensemble.""",
+        min_length=1,
     )
     data_file: str | None = Field(
         default=None,