diff --git a/gptel.el b/gptel.el
index da0178dd..ec312ad8 100644
--- a/gptel.el
+++ b/gptel.el
@@ -2281,12 +2281,14 @@ JSON query instead of the Lisp structure gptel uses."
       (buffer-disable-undo)
       (erase-buffer)
       (if (eq format 'json)
-          (progn (fundamental-mode)
-                 (insert (gptel--json-encode request-data))
-                 (json-pretty-print-buffer))
-        (lisp-data-mode)
-        (prin1 request-data)
-        (pp-buffer))
+          (progn (if (fboundp 'json-mode)
+		     (json-mode)
+		   (fundamental-mode))
+		 (insert (gptel--json-encode request-data))
+		 (json-pretty-print-buffer))
+	(lisp-data-mode)
+	(prin1 request-data)
+	(pp-buffer))
       (setq-local gptel--fsm-last request-fsm)
       (goto-char (point-min))
       (view-mode 1)