diff --git a/guix/self.scm b/guix/self.scm
index 9ff949075d..3503fbda43 100644
--- a/guix/self.scm
+++ b/guix/self.scm
@@ -588,7 +588,11 @@ (define (process-directory directory output)
                  `(#:local-build? #f              ;allow substitutes
 
                    ;; Don't annoy people about _IONBF deprecation.
-                   #:env-vars (("GUILE_WARN_DEPRECATED" . "no")))))
+                   ;; Initialize 'terminal-width' in (system repl debug)
+                   ;; to a large-enough value to make backtrace more
+                   ;; verbose.
+                   #:env-vars (("GUILE_WARN_DEPRECATED" . "no")
+                               ("COLUMNS" . "200")))))
 
 
 ;;;