diff --git a/mo b/mo
index cadb032..776b384 100755
--- a/mo
+++ b/mo
@@ -146,7 +146,7 @@ moCallFunction() {
     moArgs=()
 
     # shellcheck disable=SC2031
-    if [[ -n "$MO_ALLOW_FUNCTION_ARGUMENTS" ]]; then
+    if [[ -n "${MO_ALLOW_FUNCTION_ARGUMENTS-}" ]]; then
         moArgs=$3
     fi