diff --git a/src/lowlev1.c b/src/lowlev1.c
index 9870a1b..09b6a2c 100644
--- a/src/lowlev1.c
+++ b/src/lowlev1.c
@@ -20,8 +20,10 @@
 
 #include "lowlev1defs.h"
 
-static int mask_array[16] = {1,     3,     7,     0xf,   0x1f,   0x3f,   0x7f,   0xff,
-                      0x1ff, 0x3ff, 0x7ff, 0xfff, 0x1fff, 0x3fff, 0x7fff, 0xffff};
+static const int mask_array[16] = {
+    1,     3,     7,     0xf,   0x1f,   0x3f,   0x7f,   0xff,
+    0x1ff, 0x3ff, 0x7ff, 0xfff, 0x1fff, 0x3fff, 0x7fff, 0xffff
+};
 
 /************************************************************
  N_OP_putbitsnfd