diff --git a/scheme/std-command.scm b/scheme/std-command.scm
index acb98d3..b3aeb7d 100644
--- a/scheme/std-command.scm
+++ b/scheme/std-command.scm
@@ -55,18 +55,20 @@
     => lookup-env-var)
    (else str)))
 
-(define (expand-string str)
-  (substitute-env-vars str))
+(define (expand-string/s-expr v)
+  (if (string? v)
+      (substitute-env-vars v)
+      v))
 
 (define (expand-redirection redirection)
   (make-redirection
    (redirection-op redirection)
-   (expand-string (redirection-dest redirection))))
+   (expand-string/s-expr (redirection-dest redirection))))
 
 (define (expand-command command)
-  (let ((expanded (map expand-string (command-args command))))
+  (let ((expanded (map expand-string/s-expr (command-args command))))
     (make-command
-     (expand-string (command-executable command))
+     (expand-string/s-expr (command-executable command))
      (fold-right 
       (lambda (arg args)
 	(if (contains-glob-expression? arg)