diff --git a/src/crystal/system/wasi/file_descriptor.cr b/src/crystal/system/wasi/file_descriptor.cr
index 890e6363605c..ef71b54393f2 100644
--- a/src/crystal/system/wasi/file_descriptor.cr
+++ b/src/crystal/system/wasi/file_descriptor.cr
@@ -18,6 +18,7 @@ module Crystal::System::FileDescriptor
   end
 
   private def system_blocking_init(value)
+    self.sync = value
   end
 
   private def system_reopen(other : IO::FileDescriptor)
diff --git a/src/crystal/system/win32/file_descriptor.cr b/src/crystal/system/win32/file_descriptor.cr
index 4265701cd8b2..12fb29014fe1 100644
--- a/src/crystal/system/win32/file_descriptor.cr
+++ b/src/crystal/system/win32/file_descriptor.cr
@@ -101,6 +101,7 @@ module Crystal::System::FileDescriptor
 
   private def system_blocking_init(value)
     @system_blocking = value
+    self.sync = value
     Crystal::EventLoop.current.create_completion_port(windows_handle) unless value
   end