diff --git a/modules/home-environment.nix b/modules/home-environment.nix
index e07343034..68c572066 100644
--- a/modules/home-environment.nix
+++ b/modules/home-environment.nix
@@ -657,7 +657,7 @@ in
       destination = "/etc/profile.d/hm-session-vars.sh";
       text = ''
         # Only source this once.
-        if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+        if [ -n "''${__HM_SESS_VARS_SOURCED-}" ]; then return; fi
         export __HM_SESS_VARS_SOURCED=1
 
         ${config.lib.shell.exportAll cfg.sessionVariables}
diff --git a/modules/programs/zsh/default.nix b/modules/programs/zsh/default.nix
index be53a05d2..fed803b06 100644
--- a/modules/programs/zsh/default.nix
+++ b/modules/programs/zsh/default.nix
@@ -486,7 +486,7 @@ in
             . "${config.home.sessionVariablesPackage}/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
diff --git a/tests/modules/home-environment/session-variables.nix b/tests/modules/home-environment/session-variables.nix
index 9e5c7aa7f..9a8baed07 100644
--- a/tests/modules/home-environment/session-variables.nix
+++ b/tests/modules/home-environment/session-variables.nix
@@ -6,7 +6,7 @@ let
 
   linuxExpected = ''
     # Only source this once.
-    if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+    if [ -n "''${__HM_SESS_VARS_SOURCED-}" ]; then return; fi
     export __HM_SESS_VARS_SOURCED=1
 
     export IS_EMPTY=""
@@ -25,7 +25,7 @@ let
 
   darwinExpected = ''
     # Only source this once.
-    if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+    if [ -n "''${__HM_SESS_VARS_SOURCED-}" ]; then return; fi
     export __HM_SESS_VARS_SOURCED=1
 
     export IS_EMPTY=""
diff --git a/tests/modules/programs/zsh/session-variables.zshenv b/tests/modules/programs/zsh/session-variables.zshenv
index c132d99d6..cf99c7748 100644
--- a/tests/modules/programs/zsh/session-variables.zshenv
+++ b/tests/modules/programs/zsh/session-variables.zshenv
@@ -2,7 +2,7 @@
 . "/nix/store/00000000000000000000000000000000-hm-session-vars.sh/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
   export IS_EMPTY=""
   export IS_FALSE=false