diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 48dcbafeef..cbdff86596 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -404,6 +404,7 @@ class CacheDescriptor(_CacheDescriptorBase):
 
         wrapped.invalidate_all = cache.invalidate_all
         wrapped.cache = cache
+        wrapped.num_args = self.num_args
 
         obj.__dict__[self.orig.__name__] = wrapped
 
@@ -451,8 +452,9 @@ class CacheListDescriptor(_CacheDescriptorBase):
             )
 
     def __get__(self, obj, objtype=None):
-
-        cache = getattr(obj, self.cached_method_name).cache
+        cached_method = getattr(obj, self.cached_method_name)
+        cache = cached_method.cache
+        num_args = cached_method.num_args
 
         @functools.wraps(self.orig)
         def wrapped(*args, **kwargs):
@@ -469,12 +471,23 @@ class CacheListDescriptor(_CacheDescriptorBase):
             results = {}
             cached_defers = {}
             missing = []
-            for arg in list_args:
-                key = list(keyargs)
-                key[self.list_pos] = arg
 
+            # If the cache takes a single arg then that is used as the key,
+            # otherwise a tuple is used.
+            if num_args == 1:
+                def cache_get(arg):
+                    return cache.get(arg, callback=invalidate_callback)
+            else:
+                key = list(keyargs)
+
+                def cache_get(arg):
+                    key[self.list_pos] = arg
+                    return cache.get(tuple(key), callback=invalidate_callback)
+
+            for arg in list_args:
                 try:
-                    res = cache.get(tuple(key), callback=invalidate_callback)
+                    res = cache_get(arg)
+
                     if not isinstance(res, ObservableDeferred):
                         results[arg] = res
                     elif not res.has_succeeded():
@@ -505,17 +518,28 @@ class CacheListDescriptor(_CacheDescriptorBase):
 
                     observer = ObservableDeferred(observer)
 
-                    key = list(keyargs)
-                    key[self.list_pos] = arg
-                    cache.set(
-                        tuple(key), observer,
-                        callback=invalidate_callback
-                    )
+                    if num_args == 1:
+                        cache.set(
+                            arg, observer,
+                            callback=invalidate_callback
+                        )
 
-                    def invalidate(f, key):
-                        cache.invalidate(key)
-                        return f
-                    observer.addErrback(invalidate, tuple(key))
+                        def invalidate(f, key):
+                            cache.invalidate(key)
+                            return f
+                        observer.addErrback(invalidate, arg)
+                    else:
+                        key = list(keyargs)
+                        key[self.list_pos] = arg
+                        cache.set(
+                            tuple(key), observer,
+                            callback=invalidate_callback
+                        )
+
+                        def invalidate(f, key):
+                            cache.invalidate(key)
+                            return f
+                        observer.addErrback(invalidate, tuple(key))
 
                     res = observer.observe()
                     res.addCallback(lambda r, arg: (arg, r), arg)