diff --git a/synapse/metrics/metric.py b/synapse/metrics/metric.py
index d100841a7..7becbe049 100644
--- a/synapse/metrics/metric.py
+++ b/synapse/metrics/metric.py
@@ -180,6 +180,9 @@ class MemoryUsageMetric(object):
         self.memory_snapshots[:] = self.memory_snapshots[-max_size:]
 
     def render(self):
+        if not self.memory_snapshots:
+            return []
+
         max_rss = max(self.memory_snapshots)
         min_rss = min(self.memory_snapshots)
         sum_rss = sum(self.memory_snapshots)