diff --git a/io/fd-events.h b/io/fd-events.h
index d201a5be..71fa2e4f 100644
--- a/io/fd-events.h
+++ b/io/fd-events.h
@@ -138,7 +138,7 @@ DECLARE_MASTER_AND_CASCADING_ENGINE(select);
 DECLARE_MASTER_AND_CASCADING_ENGINE(iouring);
 DECLARE_MASTER_AND_CASCADING_ENGINE(kqueue);
 
-inline int fd_events_init(int master_engine) {
+inline int fd_events_init(uint64_t master_engine) {
     switch (master_engine) {
 #ifdef __linux__
         case INIT_EVENT_EPOLL:
diff --git a/photon.cpp b/photon.cpp
index c3162a88..25c04835 100644
--- a/photon.cpp
+++ b/photon.cpp
@@ -39,20 +39,25 @@ static thread_local uint64_t g_event_engine = 0, g_io_engine = 0;
 #define FINI_IO(name, prefix)    if (INIT_IO_##name & g_io_engine) { prefix##_fini(); }
 
 // Try to init master engine with the recommended order
+static const uint8_t recommended_order[] = {
 #if defined(__linux__)
-static const int recommended_order[] = {INIT_EVENT_EPOLL, INIT_EVENT_IOURING, INIT_EVENT_SELECT};
+    __builtin_ctz(INIT_EVENT_EPOLL), __builtin_ctz(INIT_EVENT_IOURING), __builtin_ctz(INIT_EVENT_SELECT)};
 #else   // macOS, FreeBSD ...
-static const int recommended_order[] = {INIT_EVENT_KQUEUE, INIT_EVENT_SELECT};
+    __builtin_ctz(INIT_EVENT_KQUEUE), __builtin_ctz(INIT_EVENT_SELECT)};
 #endif
 
 int __photon_init(uint64_t event_engine, uint64_t io_engine) {
     if (vcpu_init() < 0)
         return -1;
 
-    if (event_engine != INIT_EVENT_NONE) {
+    const uint64_t ALL_ENGINES = INIT_EVENT_EPOLL  |
+            INIT_EVENT_IOURING | INIT_EVENT_KQUEUE |
+            INIT_EVENT_SELECT  | INIT_EVENT_IOCP;
+    if (event_engine & ALL_ENGINES) {
         bool ok = false;
         for (auto each : recommended_order) {
-            if ((each & event_engine) && fd_events_init(each) == 0) {
+            auto x = 1ul << each;
+            if ((x & event_engine) && fd_events_init(x) == 0) {
                 ok = true;
                 break;
             }
diff --git a/test/ci-tools.cpp b/test/ci-tools.cpp
index 5453ad65..24c50fe5 100644
--- a/test/ci-tools.cpp
+++ b/test/ci-tools.cpp
@@ -73,8 +73,9 @@ int init(uint64_t event_engine, uint64_t io_engine) {
     LOG_INFO("argument specified: ` (`)", get_engine_names(arg_specified), arg_specified);
     LOG_INFO("environment specified: '`'", _engine_name);
     if (_engine && arg_specified) {
-        event_engine &= ~all_engines | _engine;
-        LOG_INFO("environment overridden: ", get_engine_names(event_engine & all_engines));
+        event_engine &= ~all_engines;
+        event_engine |= _engine;
+        LOG_INFO("event engine overridden to: ", get_engine_names(event_engine & all_engines));
     }
     return __photon_init(event_engine, io_engine);
 }