diff --git a/modules/programs/zsh.nix b/modules/programs/zsh.nix
index dcff129d..ade5a158 100644
--- a/modules/programs/zsh.nix
+++ b/modules/programs/zsh.nix
@@ -529,7 +529,7 @@ in
         . "${config.home.profileDirectory}/etc/profile.d/hm-session-vars.sh"
 
         # Only source this once
-        if [[ -z "$__HM_ZSH_SESS_VARS_SOURCED" ]]; then
+        if [[ -z "''${__HM_ZSH_SESS_VARS_SOURCED-}" ]]; then
           export __HM_ZSH_SESS_VARS_SOURCED=1
           ${envVarsStr}
         fi