diff --git a/sunspot/lib/sunspot/query/abstract_json_field_facet.rb b/sunspot/lib/sunspot/query/abstract_json_field_facet.rb
index 8c6d8c2e4..8a22f1f69 100644
--- a/sunspot/lib/sunspot/query/abstract_json_field_facet.rb
+++ b/sunspot/lib/sunspot/query/abstract_json_field_facet.rb
@@ -1,4 +1,4 @@
-require 'json' # to_params serializes the facet with to_json
+require 'json'
 
 module Sunspot
   module Query
@@ -49,7 +49,7 @@ def get_params
       end
 
       def to_params
-        { 'json.facet' => self.get_params.to_json }
+        { 'json.facet' => JSON.generate(get_params) }
       end
 
       private
diff --git a/sunspot/lib/sunspot/query/field_stats.rb b/sunspot/lib/sunspot/query/field_stats.rb
index c7189522d..0f95e8870 100644
--- a/sunspot/lib/sunspot/query/field_stats.rb
+++ b/sunspot/lib/sunspot/query/field_stats.rb
@@ -17,7 +17,7 @@ def add_json_facet(json_facet)
       def to_params
         params = {}
         if !@json_facet.nil?
-          params['json.facet'] = recursive_add_stats(@json_facet.get_params).to_json
+          params['json.facet'] = JSON.generate(recursive_add_stats(@json_facet.get_params))
         else
           params.merge!(:stats => true, :"stats.field" => [@field.indexed_name])
           params[facet_key] = @facets.map(&:indexed_name) unless @facets.empty?
diff --git a/sunspot/spec/api/nested_documents_spec.rb b/sunspot/spec/api/nested_documents_spec.rb
index f3d5fd072..07acce93e 100644
--- a/sunspot/spec/api/nested_documents_spec.rb
+++ b/sunspot/spec/api/nested_documents_spec.rb
@@ -348,6 +348,13 @@ def json_facet
       })
     end
 
+    it "serializes its params with JSON.generate, not Hash#to_json, which ActiveSupport 6.1 breaks under json 3" do
+      allow_any_instance_of(Hash).to receive(:to_json).and_raise(ArgumentError, 'unknown keyword: quirks_mode')
+
+      session.search(Project) { child_facet(:milestones, :name) }
+      expect(json_facet.keys).to eq(['name'])
+    end
+
     it 'is keyed by its :name' do
       session.search(Project) { child_facet(:milestones, :name, :name => :milestone_name) }
       expect(json_facet.keys).to eq(['milestone_name'])