diff --git a/qubesusbproxy/core3ext.py b/qubesusbproxy/core3ext.py
index 41e677d..01aa910 100644
--- a/qubesusbproxy/core3ext.py
+++ b/qubesusbproxy/core3ext.py
@@ -236,8 +236,9 @@ def _sanitize(
         # rb'USB\x202.0\x20Camera' -> 'USB 2.0 Camera'
         untrusted_device_desc = untrusted_device_desc.decode(
             'unicode_escape', errors='ignore')
+        safe_chars_set = set(safe_chars)
         return ''.join(
-            c if c in set(safe_chars) else '_' for c in untrusted_device_desc
+            c if c in safe_chars_set else '_' for c in untrusted_device_desc
         )
 
     @property