diff --git a/guix/build/emacs-build-system.scm b/guix/build/emacs-build-system.scm
index 2404dbddb..bafb1060b 100644
--- a/guix/build/emacs-build-system.scm
+++ b/guix/build/emacs-build-system.scm
@@ -93,7 +93,10 @@ store in '.el' files."
          (substitute-cmd (lambda ()
                            (substitute* (find-files "." "\\.el$")
                              (("\"/bin/([^.].*)\"" _ cmd)
-                              (string-append "\"" (which cmd) "\""))))))
+                              (let ((cmd (which cmd)))
+                                (unless cmd
+                                  (error "..."))
+                                (string-append "\"" cmd "\"")))))))
     (with-directory-excursion el-dir
       ;; Some old '.el' files (e.g., tex-buf.el in AUCTeX) are still encoded
       ;; with the "ISO-8859-1" locale.