diff --git a/src/shadowbox/scripts/make_test_certificate.sh b/src/shadowbox/scripts/make_test_certificate.sh
index d4cb59e9..35d62c9b 100755
--- a/src/shadowbox/scripts/make_test_certificate.sh
+++ b/src/shadowbox/scripts/make_test_certificate.sh
@@ -17,7 +17,7 @@
 # Make a certificate for development purposes, and populate the
 # corresponding environment variables.
 
-CERTIFICATE_NAME="$1/shadowbox-selfsigned-dev"
+readonly CERTIFICATE_NAME="$1/shadowbox-selfsigned-dev"
 export SB_CERTIFICATE_FILE="${CERTIFICATE_NAME}.crt"
 export SB_PRIVATE_KEY_FILE="${CERTIFICATE_NAME}.key"
 declare -a openssl_req_flags=(