This fix was developed inhouse on top of the ksh93 alpha version(2013-10-10-alpha).
This has been submitted to the community but not been accepted yet.
Details can be found at
https://github.com/att/ast/issues/5
--- INIT.2011-02-08.old/src/cmd/ksh93/sh/macro.c	2015-05-04 13:33:02.298225000 -0700
+++ INIT.2011-02-08/src/cmd/ksh93/sh/macro.c	2015-06-03 13:20:45.726236100 -0700
@@ -1195,6 +1195,13 @@
 		}
 		else
 			v = 0;
+		if(!v && sh_isoption(SH_NOUNSET))
+		{
+			d=fcget();
+			fcseek(-1);
+			if(d=='\0' || !strchr(":+-?=",d))
+				errormsg(SH_DICT,ERROR_exit(1),e_notset,ltos(c));
+		}
 		break;
 	    case S_ALP:
 		if(c=='.' && type==0)