diff --git a/app/controllers/admin/site_settings_controller.rb b/app/controllers/admin/site_settings_controller.rb
index ba3054abde3..e2268da25be 100644
--- a/app/controllers/admin/site_settings_controller.rb
+++ b/app/controllers/admin/site_settings_controller.rb
@@ -35,7 +35,7 @@ class Admin::SiteSettingsController < Admin::AdminController
     when :integer
       value = value.tr("^-0-9", "")
     when :file_size_restriction
-      value = value.tr("^-0-9", "")
+      value = value.tr("^0-9", "").to_i
     when :uploaded_image_list
       value = Upload.get_from_urls(value.split("|")).to_a
     end
diff --git a/app/models/topic.rb b/app/models/topic.rb
index b06cb3a1341..cc7a42a8177 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -59,7 +59,7 @@ class Topic < ActiveRecord::Base
 
   def thumbnail_info(enqueue_if_missing: false, extra_sizes: [])
     return nil unless original = image_upload
-    return nil if original.filesize >= SiteSetting.max_image_size_kb.kilobytes
+    return nil if original.filesize >= SiteSetting.max_image_size_kb.to_i.kilobytes
     return nil unless original.read_attribute(:width) && original.read_attribute(:height)
 
     infos = []
diff --git a/spec/requests/admin/site_settings_controller_spec.rb b/spec/requests/admin/site_settings_controller_spec.rb
index 2e79fd30f14..58ccd0892b7 100644
--- a/spec/requests/admin/site_settings_controller_spec.rb
+++ b/spec/requests/admin/site_settings_controller_spec.rb
@@ -276,6 +276,13 @@ RSpec.describe Admin::SiteSettingsController do
         expect(SiteSetting.suggested_topics).to eq(1000)
       end
 
+      it "sanitizes file_size_restriction values" do
+        put "/admin/site_settings/max_image_size_kb.json", params: { max_image_size_kb: "4096" }
+
+        expect(response.status).to eq(200)
+        expect(SiteSetting.max_image_size_kb).to eq(4096)
+      end
+
       it "sanitizes negative integer values correctly" do
         put "/admin/site_settings/pending_users_reminder_delay_minutes.json",
             params: {