diff --git a/django/core/files/storage.py b/django/core/files/storage.py
index b5348948fe..c3ad1faadf 100644
--- a/django/core/files/storage.py
+++ b/django/core/files/storage.py
@@ -254,8 +254,6 @@ class FileSystemStorage(Storage):
     def __init__(self, location=None, base_url=None, file_permissions_mode=None,
                  directory_permissions_mode=None):
         self._location = location
-        if base_url is not None and not base_url.endswith('/'):
-            base_url += '/'
         self._base_url = base_url
         self._file_permissions_mode = file_permissions_mode
         self._directory_permissions_mode = directory_permissions_mode
@@ -286,6 +284,8 @@ class FileSystemStorage(Storage):
 
     @cached_property
     def base_url(self):
+        if self._base_url is not None and not self._base_url.endswith('/'):
+            self._base_url += '/'
         return self._value_or_setting(self._base_url, settings.MEDIA_URL)
 
     @cached_property
diff --git a/tests/file_storage/tests.py b/tests/file_storage/tests.py
index dcd27de5c7..d419d82bd0 100644
--- a/tests/file_storage/tests.py
+++ b/tests/file_storage/tests.py
@@ -24,6 +24,7 @@ from django.test import (
     override_settings,
 )
 from django.test.utils import requires_tz_support
+from django.urls import NoReverseMatch, reverse_lazy
 from django.utils import six, timezone
 from django.utils._os import upath
 from django.utils.deprecation import RemovedInDjango20Warning
@@ -73,7 +74,7 @@ class GetStorageClassTests(SimpleTestCase):
             get_storage_class('django.core.files.non_existing_storage.NonExistingStorage')
 
 
-class FileStorageDeconstructionTests(unittest.TestCase):
+class FileSystemStorageTests(unittest.TestCase):
 
     def test_deconstruction(self):
         path, args, kwargs = temp_storage.deconstruct()
@@ -89,6 +90,14 @@ class FileStorageDeconstructionTests(unittest.TestCase):
         path, args, kwargs = storage.deconstruct()
         self.assertEqual(kwargs, kwargs_orig)
 
+    def test_lazy_base_url_init(self):
+        """
+        FileSystemStorage.__init__() shouldn't evaluate base_url.
+        """
+        storage = FileSystemStorage(base_url=reverse_lazy('app:url'))
+        with self.assertRaises(NoReverseMatch):
+            storage.url(storage.base_url)
+
 
 # Tests for TZ-aware time methods need pytz.
 requires_pytz = unittest.skipIf(pytz is None, "this test requires pytz")