diff --git a/sigma/sigma_dp.c b/sigma/sigma_dp.c
index 5adefd45..362c1e25 100644
--- a/sigma/sigma_dp.c
+++ b/sigma/sigma_dp.c
@@ -1269,7 +1269,7 @@ return SCPE_OK;
 
 /* Set unit autosize validation routine */
 
-t_stat dp_set_auto (UNIT *uptr, int32 val, char *cptr, void *desc)
+t_stat dp_set_auto (UNIT *uptr, int32 val, CONST char *cptr, void *desc)
 {
 uint32 cidx = uptr->UCTX;