diff --git a/ox-gfm.el b/ox-gfm.el
index 542eb7a..7a4408a 100644
--- a/ox-gfm.el
+++ b/ox-gfm.el
@@ -236,7 +236,7 @@ plist used as a communication channel."
                  (org-export-get-alt-title headline info) info))
          (level (1- (org-element-property :level headline)))
          (indent (concat (make-string (* level 2) ? )))
-         (anchor (or (org-element-property :custom_id headline)
+         (anchor (or (org-element-property :CUSTOM_ID headline)
                      (org-export-get-reference headline info))))
     (concat indent "- [" title "]" "(#" anchor ")")))