diff --git a/src/wuttaweb/grids/filters.py b/src/wuttaweb/grids/filters.py
index 170fbd9..54fce1c 100644
--- a/src/wuttaweb/grids/filters.py
+++ b/src/wuttaweb/grids/filters.py
@@ -189,6 +189,10 @@ class GridFilter:  # pylint: disable=too-many-instance-attributes
         self.app = self.config.get_app()
         self.label = label or self.app.make_title(self.key)
 
+        # remember original data type in case we need to revert,
+        # e.g. after changing it to 'choices' and back again
+        self.original_data_type = self.data_type
+
         # active
         self.default_active = default_active
         self.active = self.default_active
@@ -201,7 +205,7 @@ class GridFilter:  # pylint: disable=too-many-instance-attributes
         self.verb = None  # active verb is set later
 
         # choices
-        self.set_choices(choices or {})
+        self.set_choices(choices)
 
         # nullable
         self.nullable = nullable
@@ -304,7 +308,8 @@ class GridFilter:  # pylint: disable=too-many-instance-attributes
             self.data_type = "choice"
         else:
             self.choices = {}
-            self.data_type = "string"
+            if self.data_type == "choice":
+                self.data_type = self.original_data_type
 
     def normalize_choices(self, choices):
         """