diff --git a/src/os/OSSelectWakeup.cxx b/src/os/OSSelectWakeup.cxx
index 50efb44ac..d3ed3784f 100644
--- a/src/os/OSSelectWakeup.cxx
+++ b/src/os/OSSelectWakeup.cxx
@@ -240,7 +240,6 @@ void OSSelectWakeup::esp_wakeup()
 /// call from within an ISR context.
 void OSSelectWakeup::esp_wakeup_from_isr()
 {
-    AtomicHolder h(this);
     BaseType_t woken = pdFALSE;
 
     // If our VFS FD is not set in the except fd_set we can exit early.
diff --git a/src/os/OSSelectWakeup.hxx b/src/os/OSSelectWakeup.hxx
index 57f1ea708..ed45a6adf 100644
--- a/src/os/OSSelectWakeup.hxx
+++ b/src/os/OSSelectWakeup.hxx
@@ -159,6 +159,10 @@ public:
 #if OPENMRN_FEATURE_RTOS_FROM_ISR
     void wakeup_from_isr()
     {
+#if defined(ESP_PLATFORM)
+        // On multi-core ESP32s we need to lock objects even in ISRs.
+        AtomicHolder h(this);
+#endif
         pendingWakeup_ = true;
         if (inSelect_)
         {