diff --git a/modules/lang/ocaml/config.el b/modules/lang/ocaml/config.el
index 60c19ade3..696aa9822 100644
--- a/modules/lang/ocaml/config.el
+++ b/modules/lang/ocaml/config.el
@@ -111,9 +111,10 @@
     (setq +format-with 'ocp-indent)
     (when (and (executable-find "ocamlformat")
                (locate-dominating-file default-directory ".ocamlformat"))
-      (let ((ext (file-name-extension buffer-file-name t)))
-        (cond ((equal ext ".eliom")
-               (setq-local ocamlformat-file-kind 'implementation))
-              ((equal ext ".eliomi")
-               (setq-local ocamlformat-file-kind 'interface))))
+      (when buffer-file-name
+        (let ((ext (file-name-extension buffer-file-name t)))
+          (cond ((equal ext ".eliom")
+                 (setq-local ocamlformat-file-kind 'implementation))
+                ((equal ext ".eliomi")
+                 (setq-local ocamlformat-file-kind 'interface)))))
       (setq +format-with 'ocamlformat))))