diff --git a/scheme/scheme-commands.scm b/scheme/scheme-commands.scm
index c952f33..1bb04f1 100644
--- a/scheme/scheme-commands.scm
+++ b/scheme/scheme-commands.scm
@@ -6,8 +6,10 @@
             (cdr tokens))))
 
 (define (scheme-command-line? command-line)
-  (char=? (string-ref (string-trim command-line) 0) 
-          command-prefix))
+  (let ((trimmed (string-trim command-line)))
+    (and (> (string-length trimmed) 0)
+         (char=? (string-ref trimmed 0)
+                 command-prefix))))
 
 (define (eval-scheme-command command args)
   (case (string->symbol command)