diff --git a/include/WinStd/COM.h b/include/WinStd/COM.h
index 3eaceb59..f1faacbd 100644
--- a/include/WinStd/COM.h
+++ b/include/WinStd/COM.h
@@ -51,7 +51,7 @@ namespace winstd
         /// \param[in] num  COM error code
         /// \param[in] msg  Error message
         ///
-        com_runtime_error(_In_ error_type num, _In_opt_z_ const char* msg) : num_runtime_error<HRESULT>(num, std::string(msg) + ": " + message(num))
+        com_runtime_error(_In_ error_type num, _In_z_ const char* msg) : num_runtime_error<HRESULT>(num, std::string(msg) + ": " + message(num))
         {
         }
 
diff --git a/include/WinStd/Common.h b/include/WinStd/Common.h
index db493359..d94f174a 100644
--- a/include/WinStd/Common.h
+++ b/include/WinStd/Common.h
@@ -1350,7 +1350,7 @@ namespace winstd
         /// \param[in] num  Numeric error code
         /// \param[in] msg  Error message
         ///
-        num_runtime_error(_In_ error_type num, _In_opt_z_ const char *msg) :
+        num_runtime_error(_In_ error_type num, _In_z_ const char *msg) :
             m_num(num),
             runtime_error(msg)
         {}
diff --git a/include/WinStd/Sec.h b/include/WinStd/Sec.h
index 8d3e5252..2a9275d2 100644
--- a/include/WinStd/Sec.h
+++ b/include/WinStd/Sec.h
@@ -363,7 +363,7 @@ namespace winstd
         /// \param[in] num  Security provider error code
         /// \param[in] msg  Error message
         ///
-        sec_runtime_error(_In_ error_type num, _In_opt_z_ const char *msg) : num_runtime_error<SECURITY_STATUS>(num, msg)
+        sec_runtime_error(_In_ error_type num, _In_z_ const char *msg) : num_runtime_error<SECURITY_STATUS>(num, msg)
         {}
 
         ///