Fix error when trying to highlight only 1 line

Also add a test case for that.

ox-hugo.el

@@ -868,9 +868,18 @@ is in progress.  See `org-hugo--before-export-function' and
                                (dolist (param parameters)
                                  (dolist (retain-key param-keys-to-be-retained)
                                    (when (equal retain-key (car param))
-                                     (setq str (concat str " "
-                                                       (symbol-name retain-key) " "
-                                                       (cdr param))))))
+                                     (let ((val (cdr param)))
+                                       (setq str
+                                             (concat str " "
+                                                     (symbol-name retain-key) " "
+                                                     (cond
+                                                      ((stringp val)
+                                                       val)
+                                                      ((numberp val)
+                                                       (number-to-string val))
+                                                      (t
+                                                       (user-error "Invalid value %S assigned to %S"
+                                                                   val retain-key)))))))))
                                (org-string-nw-p (org-trim str))))
          ret)
     ;; (message "[ox-hugo ob-exp] info: %S" info)
@@ -2020,8 +2029,11 @@ channel."
          (parameters-str (org-element-property :parameters src-block))
          (parameters (org-babel-parse-header-arguments parameters-str))
          (hl-lines (cdr (assoc :hl_lines parameters)))
-         (hl-lines (when hl-lines
-                     (replace-regexp-in-string "," " " hl-lines))) ;"1,3-4" -> "1 3-4"
+         (hl-lines (cond
+                    ((stringp hl-lines)
+                     (replace-regexp-in-string "," " " hl-lines)) ;"1,3-4" -> "1 3-4"
+                    ((numberp hl-lines)
+                     (number-to-string hl-lines))))
          (label (let ((lbl (and (org-element-property :name src-block)
                                 (org-export-get-reference src-block info))))
                   (if lbl

test/site/content-org/all-posts.org

@@ -1147,6 +1147,15 @@ page/container width. This could be either called a bug in Hugo, or
 the HTML limitation.
 
 A workaround is below.. *use line numbers too!*.
+****** Highlighting only 1 line
+******* Org source
+#+begin_src org :noweb yes
+<<src-block-hl-without-n-just-1-line>>
+#+end_src
+******* Output
+#+begin_src org :noweb yes :exports results :results output replace :eval yes
+<<src-block-hl-without-n-just-1-line>>
+#+end_src
 **** With line numbers *not* starting from 1
 :PROPERTIES:
 :CUSTOM_ID: source-blocks-with-highlighting-with-linenums-not-starting-from-1
@@ -5528,6 +5537,14 @@ So, for emacs version 25.0.50.1, this variable will be 25_0.")
 :PROPERTIES:
 :CUSTOM_ID: source-block-with-line-highlighting-examples
 :END:
+#+begin_src org :noweb-ref src-block-hl-without-n-just-1-line
+,#+begin_src emacs-lisp :hl_lines 2
+(message "This is line 1")
+(message "This is line 2")
+(message "This is line 3")
+,#+end_src
+#+end_src
+
 #+begin_src org :noweb-ref src-block-hl-without-n
 ,#+begin_src emacs-lisp :hl_lines 1,3-5
 (message "This is line 1")

test/site/content/posts/source-block-with-highlighting.md

@@ -39,6 +39,29 @@ the HTML limitation.
 A workaround is below.. **use line numbers too!**.
 
 
+#### Highlighting only 1 line {#highlighting-only-1-line}
+
+
+##### Org source {#org-source}
+
+```org
+#+begin_src emacs-lisp :hl_lines 2
+(message "This is line 1")
+(message "This is line 2")
+(message "This is line 3")
+#+end_src
+```
+
+
+##### Output {#output}
+
+{{< highlight emacs-lisp "hl_lines=2" >}}
+(message "This is line 1")
+(message "This is line 2")
+(message "This is line 3")
+{{< /highlight >}}
+
+
 ## With line numbers **not** starting from 1 {#source-blocks-with-highlighting-with-linenums-not-starting-from-1}
 
 With line numbers enabled, the highlighting is limited to the width of

test/site/themes/bare_min

@@ -1 +1 @@
-Subproject commit 083f86e585548dbf9c0b3e00246f9daa6e87863f
+Subproject commit 4f1f3d129b1043232f15c65cf4255d6f8dd6540b