diff --git a/src/otoole/input.py b/src/otoole/input.py
index 28d747f..0b26a3f 100644
--- a/src/otoole/input.py
+++ b/src/otoole/input.py
@@ -256,15 +256,14 @@ def write(
         handle = self._header()
         logger.debug(default_values)
 
-        self.inputs = inputs # parameter/set data OR result data
+        self.inputs = inputs  # parameter/set data OR result data
         input_data = kwargs.get("input_data", None)
-        
+
         if self.write_defaults:
             try:
                 self.inputs = self._expand_defaults(inputs, default_values, input_data)
             except KeyError as ex:
                 logger.debug(f"Can not write default values due to missing {ex} data")
-                print(f"Can not write default values due to missing {ex} data")
 
         for name, df in sorted(self.inputs.items()):
             logger.debug("%s has %s columns: %s", name, len(df.index.names), df.columns)
@@ -291,13 +290,16 @@ def write(
             handle.close()
 
     def _expand_defaults(
-        self, inputs: Dict[str, pd.DataFrame], default_values: Dict[str, float], input_data: Dict[str, pd.DataFrame] = None
+        self,
+        inputs: Dict[str, pd.DataFrame],
+        default_values: Dict[str, float],
+        input_data: Dict[str, pd.DataFrame] = None,
     ) -> Dict[str, pd.DataFrame]:
         """Populates default value entry rows in dataframes
 
         Parameters
         ----------
-        inputs : Dict[str, pd.DataFrame], 
+        inputs : Dict[str, pd.DataFrame],
             param/set data or result data
         default_values : Dict[str, float]
             defaults of param/result data
@@ -311,8 +313,8 @@ def _expand_defaults(
         """
 
         sets = [x for x in self.user_config if self.user_config[x]["type"] == "set"]
-        input_data = input_data if input_data else inputs.copy() 
-        
+        input_data = input_data if input_data else inputs.copy()
+
         output = {}
         for name, data in inputs.items():
             logger.info(f"Writing defaults for {name}")
diff --git a/tests/test_input.py b/tests/test_input.py
index 6637fc0..bfee24c 100644
--- a/tests/test_input.py
+++ b/tests/test_input.py
@@ -38,6 +38,7 @@ def capital_cost():
     ).set_index(["REGION", "TECHNOLOGY", "YEAR"])
     return df
 
+
 @fixture
 def new_capacity():
     df = pd.DataFrame(
@@ -50,6 +51,7 @@ def new_capacity():
     ).set_index(["REGION", "TECHNOLOGY", "YEAR"])
     return df
 
+
 @fixture()
 def simple_default_values():
     default_values = {}
@@ -71,9 +73,8 @@ def simple_input_data(region, year, technology, capital_cost):
 
 @fixture
 def simple_result_data(new_capacity):
-    return {
-        "NewCapacity": new_capacity
-    }
+    return {"NewCapacity": new_capacity}
+
 
 @fixture
 def simple_user_config():
@@ -285,7 +286,11 @@ def test_expand_parameters_defaults(
         assert_frame_equal(actual[parameter], expected)
 
     def test_expand_result_defaults(
-        self, simple_user_config, simple_default_values, simple_input_data, simple_result_data
+        self,
+        simple_user_config,
+        simple_default_values,
+        simple_input_data,
+        simple_result_data,
     ):
         write_strategy = DummyWriteStrategy(
             user_config=simple_user_config, default_values=simple_default_values
@@ -293,7 +298,7 @@ def test_expand_result_defaults(
         actual = write_strategy._expand_defaults(
             simple_result_data, write_strategy.default_values, simple_input_data
         )
-        
+
         expected = pd.DataFrame(
             data=[
                 ["SIMPLICITY", "HYD1", 2014, 2.34],
@@ -305,9 +310,24 @@ def test_expand_result_defaults(
             ],
             columns=["REGION", "TECHNOLOGY", "YEAR", "VALUE"],
         ).set_index(["REGION", "TECHNOLOGY", "YEAR"])
-        
+
         assert_frame_equal(actual["NewCapacity"], expected)
 
+    def test_expand_results_key_error(
+        self, simple_user_config, simple_result_data, simple_default_values
+    ):
+        """When input data is just the result data"""
+        write_strategy = DummyWriteStrategy(
+            user_config=simple_user_config,
+            default_values=simple_default_values,
+            write_defaults=True,
+        )
+
+        with raises(KeyError, match="REGION"):
+            write_strategy._expand_defaults(
+                simple_result_data, write_strategy.default_values
+            )
+
 
 class TestReadStrategy:
 
@@ -531,4 +551,3 @@ def test_compare_read_to_expected_exception(self, simple_user_config, expected):
         reader = DummyReadStrategy(simple_user_config)
         with raises(OtooleNameMismatchError):
             reader._compare_read_to_expected(names=expected)
-    
\ No newline at end of file