diff --git a/lang/cem/cemcom.ansi/ch3mon.c b/lang/cem/cemcom.ansi/ch3mon.c
index 41b277403..40cc44025 100644
--- a/lang/cem/cemcom.ansi/ch3mon.c
+++ b/lang/cem/cemcom.ansi/ch3mon.c
@@ -20,7 +20,7 @@
 #include	"sizes.h"
 
 extern char options[];
-extern arith full_mask[/*MAXSIZE*/];	/* cstoper.c */
+extern arith full_mask[/*MAXSIZE + 1*/];	/* cstoper.c */
 char *symbol2str();
 
 ch3mon(oper, expp)
diff --git a/lang/cem/cemcom.ansi/ch7mon.c b/lang/cem/cemcom.ansi/ch7mon.c
index ab3e38642..f7962904b 100644
--- a/lang/cem/cemcom.ansi/ch7mon.c
+++ b/lang/cem/cemcom.ansi/ch7mon.c
@@ -18,7 +18,7 @@
 #include	"def.h"
 
 extern char options[];
-extern arith full_mask[/*MAXSIZE*/];	/* cstoper.c */
+extern arith full_mask[/*MAXSIZE + 1*/];	/* cstoper.c */
 char *symbol2str();
 
 ch7mon(oper, expp)
diff --git a/lang/cem/cemcom.ansi/cstoper.c b/lang/cem/cemcom.ansi/cstoper.c
index ddd7bf68f..c86f122be 100644
--- a/lang/cem/cemcom.ansi/cstoper.c
+++ b/lang/cem/cemcom.ansi/cstoper.c
@@ -16,7 +16,8 @@
 #include	"Lpars.h"
 #include	"assert.h"
 
-arith full_mask[MAXSIZE];/* full_mask[1] == 0XFF, full_mask[2] == 0XFFFF, .. */
+/* full_mask[1] == 0XFF, full_mask[2] == 0XFFFF, .. */
+arith full_mask[MAXSIZE + 1];
 #ifndef NOCROSS
 arith max_int;		/* maximum integer on target machine	*/
 arith max_unsigned;	/* maximum unsigned on target machine	*/
@@ -247,7 +248,7 @@ init_cst()
 
 	while (!(bt < 0))	{
 		bt = (bt << 8) + 0377, i++;
-		if (i == MAXSIZE)
+		if (i > MAXSIZE)
 			fatal("array full_mask too small for this machine");
 		full_mask[i] = bt;
 	}