diff --git a/src/testtool.c b/src/testtool.c
index a218bcc..cc8975d 100644
--- a/src/testtool.c
+++ b/src/testtool.c
@@ -418,7 +418,8 @@ void dump_fnobj(LispPTR index)
 /************************************************************************/
 
 /* Opcode names, by opcode */
-char *opcode_table[256] = {"-X-",
+static const char *opcode_table[256] = {
+                           "-X-",
                            "CAR",
                            "CDR",
                            "LISTP",