diff --git a/news/173.bugfix b/news/173.bugfix
new file mode 100644
index 0000000..8b501d8
--- /dev/null
+++ b/news/173.bugfix
@@ -0,0 +1 @@
+Don't ignore sort_on values which are not ZCatalog indexes. @davisagli
diff --git a/setup.py b/setup.py
index 5271a71..1fc1b42 100644
--- a/setup.py
+++ b/setup.py
@@ -43,7 +43,6 @@
         "plone.registry",
         "plone.uuid",
         "Products.GenericSetup",
-        "Products.ZCatalog",
         "python-dateutil",
         "Zope",
         "zope.dottedname",
diff --git a/src/plone/app/querystring/queryparser.py b/src/plone/app/querystring/queryparser.py
index 1f0a056..e6f7b14 100644
--- a/src/plone/app/querystring/queryparser.py
+++ b/src/plone/app/querystring/queryparser.py
@@ -78,12 +78,9 @@ def parseFormquery(context, formquery, sort_on=None, sort_order=None):
 
     # Add sorting (sort_on and sort_order) to the query
     if sort_on:
-        catalog = getToolByName(context, "portal_catalog")
-        # I get crazy sort_ons like '194' or 'null'.
-        if sort_on in catalog.indexes():
-            query["sort_on"] = sort_on
-            if sort_order:
-                query["sort_order"] = sort_order
+        query["sort_on"] = sort_on
+        if sort_order:
+            query["sort_order"] = sort_order
     return query
 
 
diff --git a/src/plone/app/querystring/registryreader.py b/src/plone/app/querystring/registryreader.py
index ad6e23f..1042b12 100644
--- a/src/plone/app/querystring/registryreader.py
+++ b/src/plone/app/querystring/registryreader.py
@@ -2,11 +2,8 @@
 from plone.app.querystring.interfaces import IQuerystringRegistryReader
 from plone.base.utils import safe_text
 from plone.i18n.normalizer.interfaces import IIDNormalizer
-from Products.CMFCore.utils import getToolByName
-from Products.ZCTextIndex.interfaces import IZCTextIndex
 from zope.component import getUtility
 from zope.component import queryUtility
-from zope.component.hooks import getSite
 from zope.globalrequest import getRequest
 from zope.i18n import translate
 from zope.i18nmessageid import Message
@@ -117,14 +114,9 @@ def mapOperations(self, values):
 
     def mapSortableIndexes(self, values):
         """Map sortable indexes"""
-        catalog = getToolByName(getSite(), "portal_catalog")._catalog
         sortables = {}
         for key, field in values.get("%s.field" % self.prefix).items():
-            if (
-                field["sortable"]
-                and key in catalog.indexes
-                and not IZCTextIndex.providedBy(catalog.getIndex(key))
-            ):
+            if field["sortable"]:
                 sortables[key] = values.get(f"{self.prefix}.field.{key}")
         values["sortable"] = sortables
         return values
diff --git a/src/plone/app/querystring/tests/testQueryBuilder.py b/src/plone/app/querystring/tests/testQueryBuilder.py
index 5bfcaaa..8880ed4 100644
--- a/src/plone/app/querystring/tests/testQueryBuilder.py
+++ b/src/plone/app/querystring/tests/testQueryBuilder.py
@@ -407,6 +407,10 @@ def test_munge_search_term(self):
         for _in, _out in search_term_tests:
             self.assertEqual(munge_search_term(_in), _out)
 
+    def test_query_builder_unknown_sort(self):
+        results = self.querybuilder(query=self.query, sort_on="unknown")
+        self.assertEqual(len(results), 1)
+
 
 class TestQuerybuilderResultTypes(unittest.TestCase):
     layer = TEST_PROFILE_PLONEAPPQUERYSTRING_INTEGRATION_TESTING
diff --git a/src/plone/app/querystring/tests/testQueryParser.py b/src/plone/app/querystring/tests/testQueryParser.py
index d42c08f..94ff512 100644
--- a/src/plone/app/querystring/tests/testQueryParser.py
+++ b/src/plone/app/querystring/tests/testQueryParser.py
@@ -191,7 +191,14 @@ def test_sort_on_unknown(self):
             sort_on="unknown",
             sort_order="reverse",
         )
-        self.assertEqual(parsed, {"Title": {"query": "Welcome to Plone"}})
+        self.assertEqual(
+            parsed,
+            {
+                "Title": {"query": "Welcome to Plone"},
+                "sort_on": "unknown",
+                "sort_order": "reverse",
+            },
+        )
 
     def test_path_explicit(self):
         data = {