diff --git a/scsh/rdelim.scm b/scsh/rdelim.scm
index d7ae751..aea1cae 100644
--- a/scsh/rdelim.scm
+++ b/scsh/rdelim.scm
@@ -191,7 +191,8 @@
   (let-optionals args ((port  (current-input-port))
 		       (start 0)
 		       (end   (string-length buf)))
-
+    (if (immutable? buf)
+        (error "Immutable buffer argument to %read-delimited!" buf))
     (let ((delims (x->char-set delims)))
       (let lp ((start start) (total 0))
 	(receive (terminator num-read) 
diff --git a/scsh/scsh-package.scm b/scsh/scsh-package.scm
index 33c080b..4d2da99 100644
--- a/scsh/scsh-package.scm
+++ b/scsh/scsh-package.scm
@@ -54,6 +54,7 @@
 (define-structure delimited-readers delimited-readers-interface
   (open scheme
 	byte-vectors
+        (subset primitives (immutable?))
 	signals ; ERROR
 	let-opt
 	receiving