diff --git a/scripts/install-ci-alias.sh b/scripts/install-ci-alias.sh
index c0d7004..0e0b668 100755
--- a/scripts/install-ci-alias.sh
+++ b/scripts/install-ci-alias.sh
@@ -46,9 +46,12 @@ gen_completion() {
   local out="$1" shell="$2"
   case "$shell" in
     zsh)
+      # Self-load compinit if compdef isn't defined yet (so the file works even
+      # without oh-my-zsh, or if sourced before compinit).
+      printf '%s\n' 'if ! whence compdef >/dev/null 2>&1; then autoload -Uz compinit && compinit -u; fi' > "$out"
       _CI_COMPLETE=zsh_source "$PROD_PY" -m ci_orchestrator 2>/dev/null \
         | sed -e "s#_CI_COMPLETE=zsh_complete ci)#_CI_COMPLETE=zsh_complete ${PROD_PY} -m ci_orchestrator)#" \
-              -e '/\$+commands\[ci\]/d' > "$out" ;;
+              -e '/\$+commands\[ci\]/d' >> "$out" ;;
     bash)
       _CI_COMPLETE=bash_source "$PROD_PY" -m ci_orchestrator 2>/dev/null \
         | sed -e "s#_CI_COMPLETE=bash_complete \$1#_CI_COMPLETE=bash_complete ${PROD_PY} -m ci_orchestrator#" > "$out" ;;