diff --git a/securedrop/journalist_app/forms.py b/securedrop/journalist_app/forms.py
index 3c048a9ff3..05cee4bfa5 100644
--- a/securedrop/journalist_app/forms.py
+++ b/securedrop/journalist_app/forms.py
@@ -40,7 +40,7 @@ def name_length_validation(form, field):
 def check_invalid_usernames(form, field):
     if field.data in Journalist.INVALID_USERNAMES:
         raise ValidationError(gettext(
-            "Invalid username"))
+            "This username is invalid because it is reserved for internal use by the software."))
 
 
 class NewUserForm(FlaskForm):
diff --git a/securedrop/tests/functional/journalist_navigation_steps.py b/securedrop/tests/functional/journalist_navigation_steps.py
index aaad7540e9..a237bfc104 100644
--- a/securedrop/tests/functional/journalist_navigation_steps.py
+++ b/securedrop/tests/functional/journalist_navigation_steps.py
@@ -341,7 +341,7 @@ def _admin_adds_a_user_with_invalid_username(self):
         self.wait_for(lambda: self.driver.find_element_by_css_selector(".form-validation-error"))
 
         error_msg = self.driver.find_element_by_css_selector(".form-validation-error")
-        assert "Invalid username" in error_msg.text
+        assert "This username is invalid because it is reserved for internal use by the software." in error_msg.text
 
     def _admin_adds_a_user(self, is_admin=False, new_username=""):
         self.safe_click_by_id("add-user")