diff --git a/piptools/scripts/compile.py b/piptools/scripts/compile.py
index 60d19730e..0638337f9 100755
--- a/piptools/scripts/compile.py
+++ b/piptools/scripts/compile.py
@@ -429,7 +429,11 @@ def cli(
     constraints = [
         req
         for req in constraints
-        if req.markers is None or req.markers.evaluate(dict(extra=None))
+        if req.markers is None
+        # We explicitly set extra=None to filter out optional requirements
+        # since evaluating an extra marker with no environment raises UndefinedEnvironmentName
+        # (see https://packaging.pypa.io/en/latest/markers.html#usage)
+        or req.markers.evaluate(dict(extra=None))
     ]
 
     log.debug("Using indexes:")