diff --git a/src/lib/shell/mod.rs b/src/lib/shell/mod.rs
index f5ac388972602361fe5971af2f7926f6302790e5..9395bf2c8bca903984a507275a8d63faf7a6a008 100644
--- a/src/lib/shell/mod.rs
+++ b/src/lib/shell/mod.rs
@@ -487,6 +487,9 @@ impl<'a> Shell<'a> {
                                 }
                                 Ok(())
                             }
+                            Value::Str(_) => {
+                                Err("cannot assign to an index of a string".into())
+                            }
                             _ => Ok(()),
                         }
                     }