diff --git a/flow_invitation.yaml.tmpl b/flow_invitation.yaml.tmpl
index d3f6240..db470cc 100644
--- a/flow_invitation.yaml.tmpl
+++ b/flow_invitation.yaml.tmpl
@@ -24,6 +24,18 @@ entries:
   id: invitation-enrollment-flow
   model: authentik_flows.flow
 
+### POLICIES
+- attrs:
+    expression: |
+      if not regex_match(request.context.get('prompt_data').get('username'), '\s'):
+          return True
+      ak_message("Username must not contain any whitespace!")
+      return False
+  id: username-without-spaces-policy
+  identifiers:
+    name: username-without-spaces-policy
+  model: authentik_policies_expression.expressionpolicy
+
 ### STAGES
 - identifiers:
     name: invitation-stage
@@ -41,6 +53,8 @@ entries:
       - !Find [authentik_stages_prompt.prompt, [name, default-user-settings-field-email]]
       - !Find [authentik_stages_prompt.prompt, [name, default-password-change-field-password]]
       - !Find [authentik_stages_prompt.prompt, [name, default-password-change-field-password-repeat]]
+    validation_policies:
+      - !Find [ authentik_policies_expression.expressionpolicy, [name, username-without-spaces-policy]]
 
 ### STAGE BINDINGS
 - identifiers: