diff --git a/mboxd_windows.c b/mboxd_windows.c
index 233bfdd..6a14e68 100644
--- a/mboxd_windows.c
+++ b/mboxd_windows.c
@@ -369,7 +369,7 @@
 			offset << context->block_size_shift,
 			size << context->block_size_shift,
 			cur->size << context->block_size_shift);
-		return -MBOX_R_PARAM_ERROR;
+		return -EACCES;
 	}
 
 	memset(cur->dirty_bmap + offset, val, size);