diff --git a/libs/vkd3d-shader/spirv.c b/libs/vkd3d-shader/spirv.c
index 40c3a2b4..6f64c3a5 100644
--- a/libs/vkd3d-shader/spirv.c
+++ b/libs/vkd3d-shader/spirv.c
@@ -2395,10 +2395,11 @@ static uint32_t vkd3d_dxbc_compiler_emit_input(struct vkd3d_dxbc_compiler *compi
     bool use_private_var = false;
     DWORD write_mask;
 
-    /* vThreadIDInGroupFlattened is declared with no write mask in shader
-     * bytecode generated by fxc. */
+    /* vThreadIDInGroupFlattened, vPrim are declared with no write mask in
+     * shader bytecode generated by fxc. */
     write_mask = dst->write_mask;
-    if (!write_mask && reg->type == VKD3DSPR_LOCALTHREADINDEX)
+    if (!write_mask && (reg->type == VKD3DSPR_LOCALTHREADINDEX
+            || reg->type == VKD3DSPR_PRIMID))
         write_mask = VKD3DSP_WRITEMASK_0;
 
     signature_element = vkd3d_find_signature_element_for_reg(compiler->input_signature,