diff --git a/guix/import/cran.scm b/guix/import/cran.scm
index e839af5de2..3fb2e213b0 100644
--- a/guix/import/cran.scm
+++ b/guix/import/cran.scm
@@ -231,7 +231,7 @@ (define (package->upstream-name package)
                     (start (string-rindex url #\/)))
                 ;; The URL ends on
                 ;; (string-append "/" name "_" version ".tar.gz")
-                (substring url start end)))
+                (substring url (+ start 1) end)))
              (_ #f)))
           (_ #f)))))