diff --git a/src/backend/html_markdown.py b/src/backend/html_markdown.py
index 711f7bd5..6a8aac99 100644
--- a/src/backend/html_markdown.py
+++ b/src/backend/html_markdown.py
@@ -41,10 +41,10 @@ def plugin_matrix(md):
     # Mark colour as high priority as otherwise e.g. <red>(hi) matches the
     # inline_html rule instead of the colour rule.
     md.inline.rules.insert(1, "colour")
-    if mistune.__version__.startswith("3"):
-        md.inline.register("colour", colour, parse_colour)
+    if mistune.__version__.startswith("2."):  # v2
+        md.inline.register_rule("colour", colour, parse_colour)
     else:
-        md.inline.register_rule("colour", colour, parse_colour)  # v2
+        md.inline.register("colour", colour, parse_colour)
 
     if md.renderer.NAME == "html":
         md.renderer.register("colour", render_html_colour)