diff --git a/src/os_mswin.c b/src/os_mswin.c
index cf8adda..696847e 100644
--- a/src/os_mswin.c
+++ b/src/os_mswin.c
@@ -2111,7 +2111,7 @@
 		char	*err = _(e_invexprmsg);
 		size_t	len = STRLEN(str) + STRLEN(err) + 5;
 
-		res = alloc(len);
+		res = alloc((unsigned)len);
 		if (res != NULL)
 		    vim_snprintf((char *)res, len, "%s: \"%s\"", err, str);
 		reply.dwData = COPYDATA_ERROR_RESULT;