diff --git a/src/abstract_validation_base/runner.py b/src/abstract_validation_base/runner.py
index 5a3bf3b..450cbc1 100644
--- a/src/abstract_validation_base/runner.py
+++ b/src/abstract_validation_base/runner.py
@@ -77,7 +77,8 @@ def error_summary(self) -> list[tuple[str, str]]:
         """
         errors: list[tuple[str, str]] = []
         for pydantic_err in self.pydantic_errors:
-            field_name = str(pydantic_err.get("loc", ["unknown"])[-1])
+            loc = pydantic_err.get("loc", ())
+            field_name = str(loc[-1]) if loc else "unknown"
             msg = pydantic_err.get("msg", pydantic_err.get("type", "validation_error"))
             errors.append((field_name, msg))
         if self.validator_result:
diff --git a/tests/test_runner.py b/tests/test_runner.py
index 58d9991..ab7cda1 100644
--- a/tests/test_runner.py
+++ b/tests/test_runner.py
@@ -108,6 +108,23 @@ def test_error_summary_uses_type_when_msg_missing(self) -> None:
         errors = result.error_summary
         assert errors[0][1] == "value_error"
 
+    def test_error_summary_handles_empty_loc_tuple(self) -> None:
+        """Test error_summary handles Pydantic errors with empty loc tuple.
+        
+        This can occur with model_validator failures on nested models where
+        Pydantic may return an empty location tuple.
+        """
+        result: RowResult[SampleModel] = RowResult(
+            row_index=0,
+            raw_data={},
+            pydantic_errors=[{"type": "value_error", "loc": (), "msg": "Model validation failed"}],
+        )
+
+        errors = result.error_summary
+        assert len(errors) == 1
+        assert errors[0][0] == "unknown"  # Default when loc is empty
+        assert errors[0][1] == "Model validation failed"
+
 
 # =============================================================================
 # RunnerStats Unit Tests