diff --git a/tests/admin_views/models.py b/tests/admin_views/models.py
index 0d61ba3fbb..97be6dc340 100644
--- a/tests/admin_views/models.py
+++ b/tests/admin_views/models.py
@@ -654,7 +654,8 @@ class PrePopulatedPostLargeSlug(models.Model):
     """
     title = models.CharField(max_length=100)
     published = models.BooleanField(default=False)
-    slug = models.SlugField(max_length=1000)
+    # `db_index=False` because MySQL cannot index large CharField (#21196).
+    slug = models.SlugField(max_length=1000, db_index=False)
 
 
 class AdminOrderedField(models.Model):