diff --git a/src/entity-name.ts b/src/entity-name.ts
index 5302403..ef676ca 100644
--- a/src/entity-name.ts
+++ b/src/entity-name.ts
@@ -12,6 +12,12 @@ function atLeastVersion(hass: HomeAssistant, major: number, minor: number): bool
 }
 
 function supportsEntityNames(hass: HomeAssistant): boolean {
+  // A hass can report a recent version without carrying the helper (a test
+  // harness, or a hass that has not finished initialising), and calling it
+  // then throws - so the version gate alone is not enough.
+  if (!hass || typeof (hass as { formatEntityName?: unknown }).formatEntityName !== 'function') {
+    return false;
+  }
   return atLeastVersion(hass, 2026, 4);
 }