diff --git a/src/pytest_beartype/__init__.py b/src/pytest_beartype/__init__.py
index 1d14e55..c5a3467 100644
--- a/src/pytest_beartype/__init__.py
+++ b/src/pytest_beartype/__init__.py
@@ -39,8 +39,6 @@ def pytest_configure(config: "pytest.Config") -> None:
         import sys
         from warnings import warn
 
-        stdlib_module_names = getattr(sys, "stdlib_module_names", [])
-
         class BeartypePytestWarning(BeartypeWarning):
             """
             Beartype :mod:`pytest` warning.
@@ -50,6 +48,12 @@ class BeartypePytestWarning(BeartypeWarning):
             under the active Python interpreter.
             """
 
+        # This (and `sys.builtin_module_names`) will be used to filter out
+        # already-imported modules from the warning message (about the fact that
+        # some modules have already been imported and will not be checked), whenever
+        # `--beartype-packages=*` or `--beartype-skip-packages` flags are used.
+        stdlib_module_names = getattr(sys, "stdlib_module_names", [])
+
         # Tuple of the subset of these names corresponding to previously
         # imported packages and modules under the active Python interpreter.
         if "*" in package_names: