diff --git a/src/xc.c b/src/xc.c
index 0cf536c..cd48fbe 100644
--- a/src/xc.c
+++ b/src/xc.c
@@ -175,8 +175,10 @@ int dbgflag = 0;
 
 int extended_frame; /*indicates if soft stack overflow */
 
-int n_mask_array[16] = {1,     3,     7,     0xf,   0x1f,   0x3f,   0x7f,   0xff,
-                        0x1ff, 0x3ff, 0x7ff, 0xfff, 0x1fff, 0x3fff, 0x7fff, 0xffff};
+static const int n_mask_array[16] = {
+    1,     3,     7,     0xf,   0x1f,   0x3f,   0x7f,   0xff,
+    0x1ff, 0x3ff, 0x7ff, 0xfff, 0x1fff, 0x3fff, 0x7fff, 0xffff
+};
 
 extern int TIMER_INTERVAL;