diff --git a/guix/import/cran.scm b/guix/import/cran.scm
index be34a75c8d..a94051655c 100644
--- a/guix/import/cran.scm
+++ b/guix/import/cran.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2015, 2016, 2017 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -374,7 +375,7 @@ dependencies."
                     (start (string-rindex url #\/)))
                 ;; The URL ends on
                 ;; (string-append "/" name "_" version ".tar.gz")
-                (substring url (+ start 1) end)))
+                (and start end (substring url (+ start 1) end))))
              (_ #f)))
           (_ #f)))))
 
@@ -415,6 +416,9 @@ dependencies."
 (define (cran-package? package)
   "Return true if PACKAGE is an R package from CRAN."
   (and (string-prefix? "r-" (package-name package))
+       ;; Check if the upstream name can be extracted from package uri.
+       (package->upstream-name package)
+       ;; Check if package uri(s) are prefixed by "mirror://cran".
        (match (and=> (package-source package) origin-uri)
          ((? string? uri)
           (string-prefix? "mirror://cran" uri))
diff --git a/tests/cran.scm b/tests/cran.scm
index c162d45d8a..d785ec5db1 100644
--- a/tests/cran.scm
+++ b/tests/cran.scm
@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2015 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -17,6 +18,7 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (test-cran)
+  #:use-module (gnu packages statistics)
   #:use-module (guix import cran)
   #:use-module (guix tests)
   #:use-module (srfi srfi-1)
@@ -86,6 +88,10 @@ Date/Publication: 2015-07-14 14:15:16
   '()
   ((@@ (guix import cran) listify) simple-alist "BadList"))
 
+(test-equal "r-mininal is not a cran package"
+  #f
+  ((@@ (guix import cran) cran-package?) r-minimal))
+
 (test-assert "description->package"
   ;; Replace network resources with sample data.
   (mock ((guix build download) url-fetch