diff --git a/lib/dav4rack/controller.rb b/lib/dav4rack/controller.rb
index d13de0f1..6ab64127 100644
--- a/lib/dav4rack/controller.rb
+++ b/lib/dav4rack/controller.rb
@@ -272,7 +272,7 @@ module Dav4rack
     # wait for a success/failure response.
     def lock
       depth = request.depth
-      return BadRequest unless %w[0 infinity].include?(depth)
+      return BadRequest unless request.depth == 'infinity' || request.depth == 0
 
       asked = { depth: depth }
       timeout = request.env['Timeout']