diff --git a/scooby/tracker.py b/scooby/tracker.py
index 4431e43..4e8d43f 100644
--- a/scooby/tracker.py
+++ b/scooby/tracker.py
@@ -24,7 +24,7 @@
 }
 
 
-STDLIB_PKGS = get_standard_lib_modules()
+STDLIB_PKGS = None
 
 
 def _criterion(name):
@@ -53,6 +53,8 @@ def track_imports():
     """Track all imported modules for the remainder of this session."""
     if not TRACKING_SUPPORTED:
         raise RuntimeError(SUPPORT_MESSAGE)
+    global STDLIB_PKGS
+    STDLIB_PKGS = get_standard_lib_modules()
     builtins.__import__ = scooby_import
     return