diff --git a/cores/esp8266/core_esp8266_main.cpp b/cores/esp8266/core_esp8266_main.cpp
index 04ab9f116..53f154bd0 100644
--- a/cores/esp8266/core_esp8266_main.cpp
+++ b/cores/esp8266/core_esp8266_main.cpp
@@ -88,7 +88,8 @@ extern "C" void __yield() {
 extern "C" void yield(void) __attribute__ ((weak, alias("__yield")));
 
 extern "C" void optimistic_yield() {
-    if (system_get_time() - g_micros_at_last_task_yield > OPTIMISTIC_YIELD_TIME_US)
+    if (!ETS_INTR_WITHINISR() && 
+        (system_get_time() - g_micros_at_last_task_yield) > OPTIMISTIC_YIELD_TIME_US)
     {
         __yield();
     }
diff --git a/tools/sdk/include/ets_sys.h b/tools/sdk/include/ets_sys.h
index a96e2f0be..8d7adbde3 100644
--- a/tools/sdk/include/ets_sys.h
+++ b/tools/sdk/include/ets_sys.h
@@ -61,6 +61,14 @@ typedef void (*int_handler_t)(void*);
 #define ETS_INTR_DISABLE(inum) \
     ets_isr_mask((1<<inum))
 
+inline bool ETS_INTR_WITHINISR()
+{
+    uint32_t ps;
+    __asm__ __volatile__("rsr %0,ps":"=a" (ps));
+    // PS.EXCM and PS.UM bit checks
+    return ((ps & ((1 << 4) | (1 << 5))) > 0);
+}
+
 inline uint32_t ETS_INTR_ENABLED(void)
 {
     uint32_t enabled;