diff --git a/changelog.d/9028.bugfix b/changelog.d/9028.bugfix
new file mode 100644
index 000000000..66666886a
--- /dev/null
+++ b/changelog.d/9028.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where some caches could grow larger than configured.
diff --git a/synapse/util/caches/deferred_cache.py b/synapse/util/caches/deferred_cache.py
index 601305487..1adc92eb9 100644
--- a/synapse/util/caches/deferred_cache.py
+++ b/synapse/util/caches/deferred_cache.py
@@ -105,7 +105,7 @@ class DeferredCache(Generic[KT, VT]):
             keylen=keylen,
             cache_name=name,
             cache_type=cache_type,
-            size_callback=(lambda d: len(d)) if iterable else None,
+            size_callback=(lambda d: len(d) or 1) if iterable else None,
             metrics_collection_callback=metrics_cb,
             apply_cache_factor_from_config=apply_cache_factor_from_config,
         )  # type: LruCache[KT, VT]
diff --git a/tests/util/caches/test_deferred_cache.py b/tests/util/caches/test_deferred_cache.py
index dadfabd46..ecd9efc4d 100644
--- a/tests/util/caches/test_deferred_cache.py
+++ b/tests/util/caches/test_deferred_cache.py
@@ -25,13 +25,8 @@ from tests.unittest import TestCase
 class DeferredCacheTestCase(TestCase):
     def test_empty(self):
         cache = DeferredCache("test")
-        failed = False
-        try:
+        with self.assertRaises(KeyError):
             cache.get("foo")
-        except KeyError:
-            failed = True
-
-        self.assertTrue(failed)
 
     def test_hit(self):
         cache = DeferredCache("test")
@@ -155,13 +150,8 @@ class DeferredCacheTestCase(TestCase):
         cache.prefill(("foo",), 123)
         cache.invalidate(("foo",))
 
-        failed = False
-        try:
+        with self.assertRaises(KeyError):
             cache.get(("foo",))
-        except KeyError:
-            failed = True
-
-        self.assertTrue(failed)
 
     def test_invalidate_all(self):
         cache = DeferredCache("testcache")
@@ -215,13 +205,8 @@ class DeferredCacheTestCase(TestCase):
         cache.prefill(2, "two")
         cache.prefill(3, "three")  # 1 will be evicted
 
-        failed = False
-        try:
+        with self.assertRaises(KeyError):
             cache.get(1)
-        except KeyError:
-            failed = True
-
-        self.assertTrue(failed)
 
         cache.get(2)
         cache.get(3)
@@ -239,13 +224,55 @@ class DeferredCacheTestCase(TestCase):
 
         cache.prefill(3, "three")
 
-        failed = False
-        try:
+        with self.assertRaises(KeyError):
             cache.get(2)
-        except KeyError:
-            failed = True
-
-        self.assertTrue(failed)
 
         cache.get(1)
         cache.get(3)
+
+    def test_eviction_iterable(self):
+        cache = DeferredCache(
+            "test", max_entries=3, apply_cache_factor_from_config=False, iterable=True,
+        )
+
+        cache.prefill(1, ["one", "two"])
+        cache.prefill(2, ["three"])
+
+        # Now access 1 again, thus causing 2 to be least-recently used
+        cache.get(1)
+
+        # Now add an item to the cache, which evicts 2.
+        cache.prefill(3, ["four"])
+        with self.assertRaises(KeyError):
+            cache.get(2)
+
+        # Ensure 1 & 3 are in the cache.
+        cache.get(1)
+        cache.get(3)
+
+        # Now access 1 again, thus causing 3 to be least-recently used
+        cache.get(1)
+
+        # Now add an item with multiple elements to the cache
+        cache.prefill(4, ["five", "six"])
+
+        # Both 1 and 3 are evicted since there's too many elements.
+        with self.assertRaises(KeyError):
+            cache.get(1)
+        with self.assertRaises(KeyError):
+            cache.get(3)
+
+        # Now add another item to fill the cache again.
+        cache.prefill(5, ["seven"])
+
+        # Now access 4, thus causing 5 to be least-recently used
+        cache.get(4)
+
+        # Add an empty item.
+        cache.prefill(6, [])
+
+        # 5 gets evicted and replaced since an empty element counts as an item.
+        with self.assertRaises(KeyError):
+            cache.get(5)
+        cache.get(4)
+        cache.get(6)