diff --git a/src/calliope/backend/helper_functions.py b/src/calliope/backend/helper_functions.py
index 073ef8e3..dc024796 100644
--- a/src/calliope/backend/helper_functions.py
+++ b/src/calliope/backend/helper_functions.py
@@ -809,10 +809,7 @@ def as_array(self, array: xr.DataArray, where_array: xr.DataArray) -> xr.DataArr
                   * cap_node_groups  (cap_node_groups) object 24B 'group_1' 'group_2' 'group_3'
             ```
         """
-        if (
-            self._backend_interface is not None
-            and where_array.name in self._backend_interface._dataset
-        ):
+        if self._backend_interface is not None:
             where_array = self._input_data[where_array.name]
 
         return array.where(where_array.fillna(False).astype(bool))
diff --git a/tests/test_backend_expression_parser.py b/tests/test_backend_expression_parser.py
index ded1feb9..b8baf7ec 100644
--- a/tests/test_backend_expression_parser.py
+++ b/tests/test_backend_expression_parser.py
@@ -39,7 +39,16 @@ def as_array(self, x, y):
 
 @pytest.fixture
 def valid_component_names():
-    return ["foo", "with_inf", "only_techs", "no_dims", "multi_dim_var", "no_dim_var"]
+    return [
+        "foo",
+        "with_inf",
+        "only_techs",
+        "no_dims",
+        "multi_dim_var",
+        "no_dim_var",
+        "all_true",
+        "only_techs_as_bool",
+    ]
 
 
 @pytest.fixture
@@ -567,6 +576,24 @@ def test_function_one_arg_allowed_invalid_string(
             assert check_error_or_warning(excinfo, "Expected")
 
 
+class TestEquationParserHelper:
+    @pytest.mark.parametrize(
+        ("where", "expected_notnull"),
+        [
+            ("all_true", [[True, True, True, True], [True, True, True, True]]),
+            ("only_techs_as_bool", [False, True, True, True]),
+        ],
+    )
+    def test_helper_function_where(
+        self, helper_function, eval_kwargs, where, expected_notnull
+    ):
+        """Test that `where` helper function works as expected when passed a backend interface object."""
+        string_ = f"where(no_dims, {where})"
+        parsed_ = helper_function.parse_string(string_, parse_all=True)
+        evaluated_ = parsed_[0].eval(**eval_kwargs)
+        np.testing.assert_array_equal(evaluated_.notnull(), expected_notnull)
+
+
 class TestEquationParserArithmetic:
     numbers = [2, 100, 0.02, "1e2", "2e-2", "inf"]