diff --git a/src/ert/config/design_matrix.py b/src/ert/config/design_matrix.py
index 20b5fd8df0d..b65b797c178 100644
--- a/src/ert/config/design_matrix.py
+++ b/src/ert/config/design_matrix.py
@@ -41,6 +41,12 @@ def from_config_list(cls, config_list: List[str]) -> "DesignMatrix":
             errors.append(
                 ErrorInfo("Missing required DEFAULT_SHEET").set_context(config_list)
             )
+        if design_sheet is not None and design_sheet == default_sheet:
+            errors.append(
+                ErrorInfo(
+                    "DESIGN_SHEET and DEFAULT_SHEET can not point to the same sheet."
+                ).set_context(config_list)
+            )
         if errors:
             raise ConfigValidationError.from_collected(errors)
         assert design_sheet is not None