diff --git a/roles/matrix-server/defaults/main.yml b/roles/matrix-server/defaults/main.yml
index 163901d2..e789f597 100644
--- a/roles/matrix-server/defaults/main.yml
+++ b/roles/matrix-server/defaults/main.yml
@@ -1,7 +1,7 @@
 # The bare hostname which represents your identity.
 # This is something like "example.com".
 # Note: this playbook does not touch the server referenced here.
-hostname_identity: "{{ host_specific_hostname_identity }}"
+hostname_identity: "{{ host_specific_hostname_identity|lower }}"
 
 # This is where your data lives and what we set up here.
 # This and the Riot hostname (see below) are expected to be on the same server.
diff --git a/roles/matrix-server/tasks/setup/setup_sanity_check.yml b/roles/matrix-server/tasks/setup/setup_sanity_check.yml
index a24ae1d0..bb048a20 100644
--- a/roles/matrix-server/tasks/setup/setup_sanity_check.yml
+++ b/roles/matrix-server/tasks/setup/setup_sanity_check.yml
@@ -8,4 +8,17 @@
 - name: Fail if Coturn Auth secret is missing
   fail:
     msg: "You need to set a secret in the matrix_coturn_turn_static_auth_secret variable"
-  when: "matrix_coturn_turn_static_auth_secret == ''"
\ No newline at end of file
+  when: "matrix_coturn_turn_static_auth_secret == ''"
+
+# This sanity check is only used to detect uppercase when people override these specific variables.
+#
+# If people set `host_specific_hostname_identity` without overriding other variables (the general use-case),
+# we take care to lower-case it automatically and it won't cause trouble anyway.
+- name: Fail if uppercase domain used
+  fail:
+    msg: "Detected that you're using an uppercase domain name - `{{ item }}`. This will cause trouble. Please use all-lowercase!"
+  when: "item != item|lower"
+  with_items:
+    - "{{ hostname_identity }}"
+    - "{{ hostname_matrix }}"
+    - "{{ hostname_riot }}"
\ No newline at end of file