diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 22902f0..fc0ad3a 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -4,6 +4,7 @@ Changelog
 * Unreleased
     * Allowed for rectangular avatars. Custom avatar tag templates now require the specification of both a ``width`` and ``height`` attribute instead of ``size``.
     * Made ``True`` the default value of ``AVATAR_CLEANUP_DELETED``. (Set to ``False`` to obtain previous behavior).
+    * Fix invalidate_cache for on-the-fly created thumbnails
 
 * 6.0.1 (August 12, 2022)
     * Exclude tests folder from distribution.
diff --git a/avatar/models.py b/avatar/models.py
index 8a625f1..a3e7413 100644
--- a/avatar/models.py
+++ b/avatar/models.py
@@ -188,6 +188,7 @@ class Avatar(models.Model):
             thumb = self.avatar.storage.save(
                 self.avatar_name(width, height), thumb_file
             )
+        invalidate_cache(self.user, width, height)
 
     def avatar_url(self, width, height=None):
         return self.avatar.storage.url(self.avatar_name(width, height))
diff --git a/avatar/utils.py b/avatar/utils.py
index 2522081..51776b8 100644
--- a/avatar/utils.py
+++ b/avatar/utils.py
@@ -28,13 +28,17 @@ def get_user(userdescriptor):
     return User.objects.get_by_natural_key(userdescriptor)
 
 
-def get_cache_key(user_or_username, width, height, prefix):
+def get_cache_key(user_or_username, prefix, width=None, height=None):
     """
     Returns a cache key consisten of a username and image size.
     """
     if isinstance(user_or_username, get_user_model()):
         user_or_username = get_username(user_or_username)
-    key = "%s_%s_%s_%s" % (prefix, user_or_username, width, height or width)
+    key = f"{prefix}_{user_or_username}"
+    if width:
+        key += f"_{width}"
+    if height or width:
+        key += f"x{height or width}"
     return "%s_%s" % (
         slugify(key)[:100],
         hashlib.md5(force_bytes(key)).hexdigest(),
@@ -62,11 +66,16 @@ def cache_result(default_size=settings.AVATAR_DEFAULT_SIZE):
         def cached_func(user, width=None, height=None, **kwargs):
             prefix = func.__name__
             cached_funcs.add(prefix)
-            key = get_cache_key(user, width or default_size, height, prefix=prefix)
+            key = get_cache_key(user, prefix, width or default_size, height)
             result = cache.get(key)
             if result is None:
                 result = func(user, width or default_size, height, **kwargs)
                 cache_set(key, result)
+                # add image size to set of cached sizes so we can invalidate them later
+                sizes_key = get_cache_key(user, "cached_sizes")
+                sizes = cache.get(sizes_key, set())
+                sizes.add((width or default_size, height or width or default_size))
+                cache_set(sizes_key, sizes)
             return result
 
         return cached_func
@@ -78,16 +87,18 @@ def invalidate_cache(user, width=None, height=None):
     """
     Function to be called when saving or changing a user's avatars.
     """
-    sizes = set(settings.AVATAR_AUTO_GENERATE_SIZES)
+    sizes_key = get_cache_key(user, "cached_sizes")
+    sizes = cache.get(sizes_key, set())
     if width is not None:
         sizes.add((width, height or width))
     for prefix in cached_funcs:
         for size in sizes:
             if isinstance(size, int):
-                cache.delete(get_cache_key(user, size, size, prefix))
+                cache.delete(get_cache_key(user, prefix, size))
             else:
                 # Size is specified with height and width.
-                cache.delete(get_cache_key(user, size[0], size[1], prefix))
+                cache.delete(get_cache_key(user, prefix, size[0], size[1]))
+    cache.set(sizes_key, set())
 
 
 def get_default_avatar_url():
diff --git a/tests/tests.py b/tests/tests.py
index a99fb75..cce39ac 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -5,6 +5,7 @@ from shutil import rmtree
 
 from django.contrib.admin.sites import AdminSite
 from django.core import management
+from django.core.cache import cache
 from django.test import TestCase
 from django.test.utils import override_settings
 from django.urls import reverse
@@ -15,7 +16,12 @@ from avatar.conf import settings
 from avatar.models import Avatar
 from avatar.signals import avatar_deleted
 from avatar.templatetags import avatar_tags
-from avatar.utils import get_primary_avatar, get_user_model
+from avatar.utils import (
+    get_cache_key,
+    get_primary_avatar,
+    get_user_model,
+    invalidate_cache,
+)
 
 
 class AssertSignal:
@@ -465,10 +471,23 @@ class AvatarTests(TestCase):
         self.assertMediaFileExists(avatar_80_url)
         self.assertNotEqual(avatar_80_mtime, self.get_media_file_mtime(avatar_80_url))
 
-    # def testAvatarOrder
-    # def testReplaceAvatarWhenMaxIsOne
-    # def testHashFileName
-    # def testHashUserName
-    # def testChangePrimaryAvatar
-    # def testDeleteThumbnailAndRecreation
-    # def testAutomaticThumbnailCreation
+    def test_invalidate_cache(self):
+        upload_helper(self, "test.png")
+        sizes_key = get_cache_key(self.user, "cached_sizes")
+        sizes = cache.get(sizes_key, set())
+        # Only default 80x80 thumbnail is cached
+        self.assertEqual(len(sizes), 1)
+        # Invalidate cache
+        invalidate_cache(self.user)
+        sizes = cache.get(sizes_key, set())
+        # No thumbnail is cached.
+        self.assertEqual(len(sizes), 0)
+        # Create a custom 25x25 thumbnail and check that it is cached
+        avatar_tags.avatar(self.user, 25)
+        sizes = cache.get(sizes_key, set())
+        self.assertEqual(len(sizes), 1)
+        # Invalidate cache again.
+        invalidate_cache(self.user)
+        sizes = cache.get(sizes_key, set())
+        # It should now be empty again
+        self.assertEqual(len(sizes), 0)