diff --git a/helpers/apps b/helpers/apps
index c5fe6cdad..bb60fea59 100644
--- a/helpers/apps
+++ b/helpers/apps
@@ -126,9 +126,6 @@ ynh_load_app_environment() {
         # Manage arguments with getopts
         ynh_handle_getopts_args "$@"
 
-        # Retrieve the list of installed apps
-        local installed_apps_list=($(yunohost app list --output-as json --quiet | jq -r .apps[].id))
-
         # Force Bash to be used to run this helper
         if [ $0 != "bash" ]
         then
@@ -137,14 +134,21 @@ ynh_load_app_environment() {
         fi
 
         # Make sure the app is installed
+        local installed_apps_list=($(yunohost app list --output-as json --quiet | jq -r .apps[].id))
         if [[ " ${installed_apps_list[*]} " != *" ${app} "* ]]
         then
             ynh_print_err --message="$app is not in the apps list"
             exit 1
         fi
 
+        # Make sure the app is installed
+        if ! id -u "$app" &>/dev/null; then
+            ynh_print_err --message="There is no \"$app\" system user"
+            exit 1
+        fi
+
         # Make sure the app has an install_dir setting
-        install_dir="$(yunohost app setting $app install_dir)"
+        local install_dir="$(yunohost app setting $app install_dir)"
         if [ -z "$install_dir" ]
         then
             ynh_print_err --message="$app has no install_dir setting (does it use packaging format >=2?)"
@@ -152,18 +156,21 @@ ynh_load_app_environment() {
         fi
 
         # Load the Environment variables from the app's service
-        env_var=`systemctl show $app.service -p "Environment" --value`
+        local env_var=`systemctl show $app.service -p "Environment" --value`
         [ -n "$env_var" ] && export $env_var;
         export HOME=$install_dir;
 
         # Source the EnvironmentFiles from the app's service
-        env_files=(`systemctl show $app.service -p "EnvironmentFiles" --value`)
+        local env_files=(`systemctl show $app.service -p "EnvironmentFiles" --value`)
         if [ ${#env_files[*]} -gt 0 ]
         then
+            # set -/+a enables and disables new variables being automatically exported. Needed when using `source`.
+            set -a
             for file in ${env_files[*]}
             do
                 [[ $file = /* ]] && source $file
             done
+            set +a
         fi
 
         # Open the app shell