diff --git a/lib/nghttp2_helper.c b/lib/nghttp2_helper.c
index 3b282c73..81a8a0cf 100644
--- a/lib/nghttp2_helper.c
+++ b/lib/nghttp2_helper.c
@@ -340,7 +340,7 @@ const char *nghttp2_strerror(int error_code) {
 }
 
 /* Generated by gennmchartbl.py */
-static int VALID_HD_NAME_CHARS[] = {
+static const int VALID_HD_NAME_CHARS[] = {
     0 /* NUL  */, 0 /* SOH  */, 0 /* STX  */, 0 /* ETX  */,
     0 /* EOT  */, 0 /* ENQ  */, 0 /* ACK  */, 0 /* BEL  */,
     0 /* BS   */, 0 /* HT   */, 0 /* LF   */, 0 /* VT   */,
@@ -428,7 +428,7 @@ int nghttp2_check_header_name(const uint8_t *name, size_t len) {
 }
 
 /* Generated by genvchartbl.py */
-static int VALID_HD_VALUE_CHARS[] = {
+static const int VALID_HD_VALUE_CHARS[] = {
     0 /* NUL  */, 0 /* SOH  */, 0 /* STX  */, 0 /* ETX  */,
     0 /* EOT  */, 0 /* ENQ  */, 0 /* ACK  */, 0 /* BEL  */,
     0 /* BS   */, 1 /* HT   */, 0 /* LF   */, 0 /* VT   */,