diff --git a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/modem/ModemPeripheral.java b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/modem/ModemPeripheral.java
index 45be424a3..7b1cf5dfa 100644
--- a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/modem/ModemPeripheral.java
+++ b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/modem/ModemPeripheral.java
@@ -4,6 +4,7 @@
 
 package dan200.computercraft.shared.peripheral.modem;
 
+import com.google.errorprone.annotations.concurrent.GuardedBy;
 import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
 import dan200.computercraft.api.network.Packet;
@@ -85,7 +86,7 @@ import java.util.Set;
  */
 public abstract class ModemPeripheral implements IPeripheral, PacketSender, PacketReceiver {
     private @Nullable PacketNetwork network;
-    private final Set<IComputerAccess> computers = new HashSet<>(1);
+    private final @GuardedBy("computers") Set<IComputerAccess> computers = new HashSet<>(1);
     private final ModemState state;
 
     protected ModemPeripheral(ModemState state) {
diff --git a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java
index be2feee1e..3f89c17cd 100644
--- a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java
+++ b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java
@@ -25,8 +25,9 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import javax.annotation.Nullable;
-import java.util.HashSet;
+import java.util.Collections;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Consumer;
 
 public class MonitorBlockEntity extends BlockEntity {
@@ -46,7 +47,7 @@ public class MonitorBlockEntity extends BlockEntity {
     private @Nullable ServerMonitor serverMonitor;
     private @Nullable ClientMonitor clientMonitor;
     private @Nullable MonitorPeripheral peripheral;
-    private final Set<IComputerAccess> computers = new HashSet<>();
+    private final Set<IComputerAccess> computers = Collections.newSetFromMap(new ConcurrentHashMap<>());
 
     private boolean needsUpdate = false;
     private boolean needsValidating = false;
diff --git a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/speaker/SpeakerPeripheral.java b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/speaker/SpeakerPeripheral.java
index c68e4ad71..4a6ab7ef0 100644
--- a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/speaker/SpeakerPeripheral.java
+++ b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/speaker/SpeakerPeripheral.java
@@ -4,6 +4,7 @@
 
 package dan200.computercraft.shared.peripheral.speaker;
 
+import com.google.errorprone.annotations.concurrent.GuardedBy;
 import dan200.computercraft.api.lua.ILuaContext;
 import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.lua.LuaFunction;
@@ -57,7 +58,7 @@ public abstract class SpeakerPeripheral implements IPeripheral {
     public static final int SAMPLE_RATE = 48000;
 
     private final UUID source = UUID.randomUUID();
-    private final Set<IComputerAccess> computers = new HashSet<>();
+    private final @GuardedBy("computers") Set<IComputerAccess> computers = new HashSet<>();
 
     private long clock = 0;
     private long lastPositionTime;