diff --git a/lib/url-guard.sh b/lib/url-guard.sh
index 587c01b..7e59f44 100644
--- a/lib/url-guard.sh
+++ b/lib/url-guard.sh
@@ -41,9 +41,14 @@ _rest_charset_ok() ( LC_ALL=C; case "$1" in
 # Literal local/private/metadata targets. This is a LITERAL check, not a DNS
 # one: it stops the obvious, not a hostname that resolves inward.
 _host_is_local() ( LC_ALL=C
-  # ${1,,} not tr: no fork, and no SC2018/SC2019 noise. Safe because the
-  # charset guard has already run — the string is [A-Za-z0-9.-] by here.
-  case "${1,,}" in
+  # `nocasematch` not ${1,,}: the lowercase expansion is bash 4.0+, and macOS
+  # ships bash 3.2 as /bin/bash — there it raised "bad substitution" and the
+  # subshell exited 1, i.e. "not local", so EVERY local/private/metadata host
+  # was allowed through. Keeps the no-fork property the lowercase form had.
+  # Safe because the charset guard has already run — the string is
+  # [A-Za-z0-9.-] by here, and LC_ALL=C keeps the folding ASCII-only.
+  shopt -s nocasematch
+  case "$1" in
     localhost|*.localhost|*.local|0.0.0.0|broadcasthost) exit 0 ;;
     127.*|10.*|169.254.*|192.168.*) exit 0 ;;
     172.1[6-9].*|172.2[0-9].*|172.3[01].*) exit 0 ;;