diff --git a/lib/plat/freertos/private-lib-plat-freertos.h b/lib/plat/freertos/private-lib-plat-freertos.h
index 089222392..4d7846910 100644
--- a/lib/plat/freertos/private-lib-plat-freertos.h
+++ b/lib/plat/freertos/private-lib-plat-freertos.h
@@ -119,7 +119,8 @@ struct lws;
 int
 insert_wsi(const struct lws_context *context, struct lws *wsi);
 
-#define delete_from_fd(A,B) A->lws_lookup[B - lws_plat_socket_offset()] = 0
+#define delete_from_fd(A,B) assert((int)A->max_fds > B - lws_plat_socket_offset()); \
+    A->lws_lookup[B - lws_plat_socket_offset()] = 0
 
 #define LWS_PLAT_TIMER_TYPE		TimerHandle_t
 #define LWS_PLAT_TIMER_CB(name, var)	void name(TimerHandle_t var)
diff --git a/lib/plat/optee/private-lib-plat-optee.h b/lib/plat/optee/private-lib-plat-optee.h
index 9fb570fa0..7d2b351af 100644
--- a/lib/plat/optee/private-lib-plat-optee.h
+++ b/lib/plat/optee/private-lib-plat-optee.h
@@ -46,5 +46,6 @@
 				  lws_plat_socket_offset()] == 0); \
 				 A->lws_lookup[B->desc.sockfd - \
 				  lws_plat_socket_offset()] = B
-#define delete_from_fd(A,B) A->lws_lookup[B - lws_plat_socket_offset()] = 0
+#define delete_from_fd(A,B)  assert((int)A->max_fds > B - lws_plat_socket_offset()); \
+    A->lws_lookup[B - lws_plat_socket_offset()] = 0
 
diff --git a/lib/plat/unix/unix-fds.c b/lib/plat/unix/unix-fds.c
index d6cd64d2e..6ef4ac3fc 100644
--- a/lib/plat/unix/unix-fds.c
+++ b/lib/plat/unix/unix-fds.c
@@ -174,8 +174,10 @@ delete_from_fd(const struct lws_context *context, int fd)
 	struct lws **p, **done;
 
 	if (!context->max_fds_unrelated_to_ulimit) {
-		if (context->lws_lookup)
+		if (context->lws_lookup) {
+			assert((int)context->max_fds > fd - lws_plat_socket_offset());
 			context->lws_lookup[fd - lws_plat_socket_offset()] = NULL;
+		}
 
 		return;
 	}
diff --git a/minimal-examples/embedded/esp32/esp-c3dev/private-lib-plat-freertos.h b/minimal-examples/embedded/esp32/esp-c3dev/private-lib-plat-freertos.h
index a81bbc055..25fc17e41 100644
--- a/minimal-examples/embedded/esp32/esp-c3dev/private-lib-plat-freertos.h
+++ b/minimal-examples/embedded/esp32/esp-c3dev/private-lib-plat-freertos.h
@@ -116,7 +116,8 @@ struct lws;
 int
 insert_wsi(const struct lws_context *context, struct lws *wsi);
 
-#define delete_from_fd(A,B) A->lws_lookup[B - lws_plat_socket_offset()] = 0
+#define delete_from_fd(A,B)  assert((int)A->max_fds > B - lws_plat_socket_offset()); \
+    A->lws_lookup[B - lws_plat_socket_offset()] = 0
 
 #define LWS_PLAT_TIMER_TYPE		TimerHandle_t
 #define LWS_PLAT_TIMER_CB(name, var)	void name(TimerHandle_t var)