diff --git a/flow_invitation.yaml.tmpl b/flow_invitation.yaml.tmpl
index d3f6240..ccdac53 100644
--- a/flow_invitation.yaml.tmpl
+++ b/flow_invitation.yaml.tmpl
@@ -24,6 +24,24 @@ entries:
   id: invitation-enrollment-flow
   model: authentik_flows.flow
 
+### POLICIES
+- attrs:
+    expression: "if not regex_match(request.context.get('prompt_data').get('username'),\
+      \ '\\s'):\n    return True\n\nak_message(\"Username must not contain\
+      \ whitespace!\")\nreturn False"
+    name: username-without-spaces-policy
+  id: username-without-spaces-policy
+  identifiers:
+    name: username-without-spaces-policy
+  model: authentik_policies_expression.expressionpolicy
+
+### POLICY BINDINGS
+- identifiers:
+    policy: !KeyOf username-without-spaces-policy
+    target: !KeyOf prompt-stage-binding
+    order: 10
+  model: authentik_policies.policybinding
+
 ### STAGES
 - identifiers:
     name: invitation-stage
@@ -53,6 +71,7 @@ entries:
     stage: !KeyOf enrollment-prompt-userdata
     target: !KeyOf invitation-enrollment-flow
   model: authentik_flows.flowstagebinding
+  id: prompt-stage-binding
 - identifiers:
     order: 20
     stage: !Find [authentik_stages_user_write.userwritestage, [name,  default-source-enrollment-write]]
diff --git a/flow_translation.yaml.tmpl b/flow_translation.yaml.tmpl
index 2baaa3f..0c709b6 100644
--- a/flow_translation.yaml.tmpl
+++ b/flow_translation.yaml.tmpl
@@ -69,3 +69,16 @@ entries:
   attrs:
     label: !Context transl_username
     placeholder: !Context transl_username
+
+### POLICIES
+- model: authentik_policies_expression.expressionpolicy
+  identifiers:
+    name: username-without-spaces-policy
+  attrs:
+    expression: "if not regex_match(request.context.get('prompt_data').get('username'),\
+      \ '\\s'):\n    return True\n\nak_message(\"Benutzername darf kein Leerzeichen\
+      \ enthalten\")\nreturn False"
+      name: username-without-spaces-policy
+  id: username-without-spaces-policy
+
+Benutzername darf kein Leerzeichen enthalten\")\n
\ No newline at end of file