diff --git a/_pytest/nodes.py b/_pytest/nodes.py
index dfe63b220..33752064f 100644
--- a/_pytest/nodes.py
+++ b/_pytest/nodes.py
@@ -329,7 +329,7 @@ class FSCollector(Collector):
 
             if not nodeid:
                 nodeid = _check_initialpaths_for_relpath(session, fspath)
-            if os.sep != SEP:
+            if nodeid and os.sep != SEP:
                 nodeid = nodeid.replace(os.sep, SEP)
 
         super(FSCollector, self).__init__(name, parent, config, session, nodeid=nodeid, fspath=fspath)