diff --git a/libs/vkd3d-shader/vkd3d_shader_main.c b/libs/vkd3d-shader/vkd3d_shader_main.c
index 664110e7..6289a185 100644
--- a/libs/vkd3d-shader/vkd3d_shader_main.c
+++ b/libs/vkd3d-shader/vkd3d_shader_main.c
@@ -1152,5 +1152,8 @@ int vkd3d_shader_preprocess(const struct vkd3d_shader_compile_info *compile_info
 {
     TRACE("compile_info %p, out %p, messages %p.\n", compile_info, out, messages);
 
+    if (messages)
+        *messages = NULL;
+
     return VKD3D_ERROR_NOT_IMPLEMENTED;
 }