diff --git a/labthings/core/event.py b/labthings/core/event.py
index a6fcc23b..778360a9 100644
--- a/labthings/core/event.py
+++ b/labthings/core/event.py
@@ -4,6 +4,7 @@
 import logging
 
 from gevent.monkey import get_original
+from gevent.lock import Semaphore
 
 # Guarantee that Task threads will always be proper system threads, regardless of Gevent patches
 Event = get_original("threading", "Event")
@@ -19,6 +20,7 @@ class ClientEvent(object):
 
     def __init__(self):
         self.events = {}
+        self._setting_lock = Semaphore(value=1)
 
     def wait(self, timeout: int = 5):
         """Wait for the next data frame (invoked from each client's thread)."""
@@ -40,23 +42,24 @@ def wait(self, timeout: int = 5):
 
     def set(self, timeout=5):
         """Signal that a new frame is available."""
-        now = time.time()
-        remove = None
-        for ident, event in self.events.items():
-            if not event[0].is_set():
-                # if this client's event is not set, then set it
-                # also update the last set timestamp to now
-                event[0].set()
-                event[1] = now
-            else:
-                # if the client's event is already set, it means the client
-                # did not process a previous frame
-                # if the event stays set for more than `timeout` seconds, then assume
-                # the client is gone and remove it
-                if now - event[1] >= timeout:
-                    remove = ident
-        if remove:
-            del self.events[remove]
+        with self._setting_lock:
+            now = time.time()
+            remove_keys = set()
+            for ident, event in self.events.items():
+                if not event[0].is_set():
+                    # if this client's event is not set, then set it
+                    # also update the last set timestamp to now
+                    event[0].set()
+                    event[1] = now
+                else:
+                    # if the client's event is already set, it means the client
+                    # did not process a previous frame
+                    # if the event stays set for more than `timeout` seconds, then
+                    # assume the client is gone and remove it
+                    if now - event[1] >= timeout:
+                        remove_keys.add(ident)
+            if remove_keys:
+                del self.events[ident]
 
     def clear(self):
         """Clear frame event, once processed."""