diff --git a/modules/rostests/winetests/riched20/txtsrv.c b/modules/rostests/winetests/riched20/txtsrv.c
index 0ef617b18d2..1c172277f1d 100644
--- a/modules/rostests/winetests/riched20/txtsrv.c
+++ b/modules/rostests/winetests/riched20/txtsrv.c
@@ -130,6 +130,7 @@ static ULONG WINAPI ITextHostImpl_Release(ITextHost *iface)
 }
 
 #ifdef __REACTOS__
+#undef __thiscall
 #define __thiscall WINAPI
 #endif /* __REACTOS__ */