diff --git a/server/data_cxg/cxg_adaptor.py b/server/data_cxg/cxg_adaptor.py
index bbdd94c3..968be4eb 100644
--- a/server/data_cxg/cxg_adaptor.py
+++ b/server/data_cxg/cxg_adaptor.py
@@ -237,6 +237,13 @@ class CxgAdaptor(DataAdaptor):
     def get_X_array(self, obs_mask=None, var_mask=None):
         obs_items = pack_selector_from_mask(obs_mask)
         var_items = pack_selector_from_mask(var_mask)
+        if obs_items is None or var_items is None:
+            # If either zero rows or zero columns were selected, return an empty 2d array.
+            shape = self.get_shape()
+            obs_size = 0 if obs_items is None else shape[0] if obs_mask is None else np.count_nonzero(obs_mask)
+            var_size = 0 if var_items is None else shape[1] if var_mask is None else np.count_nonzero(var_mask)
+            return np.ndarray((obs_size, var_size))
+
         X = self.open_array("X")
 
         if X.schema.sparse:
diff --git a/server/data_cxg/cxg_util.py b/server/data_cxg/cxg_util.py
index 26ef2297..f5759e8d 100644
--- a/server/data_cxg/cxg_util.py
+++ b/server/data_cxg/cxg_util.py
@@ -20,7 +20,7 @@ def pack_selector_from_mask(boolarray):
 def pack_selector_from_indices(selector):
 
     if len(selector) == 0:
-        return slice(None)
+        return None
 
     result = []
     current = slice(selector[0], selector[0])
diff --git a/server/test/test_api.py b/server/test/test_api.py
index 319a5847..4db3aea5 100644
--- a/server/test/test_api.py
+++ b/server/test/test_api.py
@@ -244,6 +244,19 @@ class EndPoints(object):
         self.assertEqual(df["n_rows"], 2638)
         self.assertEqual(df["n_cols"], 1)
 
+    def test_data_get_unknown_filter_fbs(self):
+        index_col_name = self.schema["schema"]["annotations"]["var"]["index"]
+        endpoint = "data/var"
+        query = f"var:{index_col_name}=UNKNOWN"
+        url = f"{self.URL_BASE}{endpoint}?{query}"
+        header = {"Accept": "application/octet-stream"}
+        result = self.session.get(url, headers=header)
+        self.assertEqual(result.status_code, HTTPStatus.OK)
+        self.assertEqual(result.headers["Content-Type"], "application/octet-stream")
+        df = decode_fbs.decode_matrix_FBS(result.content)
+        self.assertEqual(df["n_rows"], 2638)
+        self.assertEqual(df["n_cols"], 0)
+
     def test_data_put_single_var(self):
         endpoint = "data/var"
         url = f"{self.URL_BASE}{endpoint}"