diff --git a/mastodon/Mastodon.py b/mastodon/Mastodon.py
index 874d925..8b7064f 100644
--- a/mastodon/Mastodon.py
+++ b/mastodon/Mastodon.py
@@ -929,7 +929,8 @@ class Mastodon:
 
         # Validate visibility parameter
         valid_visibilities = ['private', 'public', 'unlisted', 'direct', '']
-        if params_initial['visibility'].lower() not in valid_visibilities:
+        params_initial['visibility'] = params_initial['visibility'].lower()
+        if params_initial['visibility'] not in valid_visibilities:
             raise ValueError('Invalid visibility value! Acceptable '
                              'values are %s' % valid_visibilities)