diff --git a/src/vim.h b/src/vim.h
index 1bd3821..abee386 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1715,7 +1715,7 @@
 typedef void	    *vim_acl_T;		/* dummy to pass an ACL to a function */
 
 #ifndef mch_memmove
-# define mch_memmove(to, from, len) memmove((char*)(to), (char*)(from), (char*)(len))
+# define mch_memmove(to, from, len) memmove((char*)(to), (char*)(from), (size_t)(len))
 #endif
 
 /*