diff --git a/gen/typinf.cpp b/gen/typinf.cpp
index 1942c543..566b13d6 100644
--- a/gen/typinf.cpp
+++ b/gen/typinf.cpp
@@ -716,8 +716,11 @@ void TypeInfoStructDeclaration::llvmDefine()
 
     ClassDeclaration* tscd = Type::typeinfostruct;
 
-    assert((!global.params.is64bit && tscd->fields.dim == 11) ||
-           (global.params.is64bit && tscd->fields.dim == 13));
+    // On Windows x86_64, class TypeInfo_Struct contains 2 additional fields
+    // (m_arg1/m_arg2) which are used for the X86_64 System V ABI varargs 
+    // implementation. They are not present on any other os.
+    assert((!(global.params.is64bit && global.params.os == OSWindows) && tscd->fields.dim == 11) ||
+           (global.params.is64bit && global.params.os == OSWindows && tscd->fields.dim == 13));
 
     //void function(void*)                    xdtor;
     b.push_funcptr(sd->dtor);