diff --git a/src/docgen/ai_client.py b/src/docgen/ai_client.py
index 65a9f62..3a4cd10 100644
--- a/src/docgen/ai_client.py
+++ b/src/docgen/ai_client.py
@@ -320,14 +320,27 @@ def resolve_image_model(model: str, settings: AISettings | None = None, *, cfg:
     return chosen
 
 
+def _grok_voice_name(chosen: str) -> str:
+    """Map a voice onto Grok. An unknown name must not silently become ``eve``."""
+    key = chosen.lower()
+    if not key:
+        return DEFAULT_GROK_TTS_VOICE
+    if key in _GROK_VOICES:
+        return key
+    mapped = _OPENAI_TO_GROK_VOICE.get(key)
+    if mapped is None:
+        raise AIError(
+            f"voice {chosen!r} is not a Grok voice and has no OpenAI alias"
+        )
+    return mapped
+
+
 def resolve_tts_voice(voice: str, settings: AISettings | None = None, *, cfg: "Config | None" = None) -> str:
     chosen = (voice or "").strip()
     st = settings or resolve_ai_settings(cfg)
     if not st.is_grok:
         return chosen
-    if chosen.lower() in _GROK_VOICES:
-        return chosen.lower()
-    return _OPENAI_TO_GROK_VOICE.get(chosen.lower(), DEFAULT_GROK_TTS_VOICE)
+    return _grok_voice_name(chosen)
 
 
 def chat_completion(
diff --git a/tests/test_ai_client.py b/tests/test_ai_client.py
index 6049ec2..a1427a6 100644
--- a/tests/test_ai_client.py
+++ b/tests/test_ai_client.py
@@ -75,6 +75,16 @@ def test_xai_alias_and_openai_model_remap(
     assert resolve_tts_voice("eve", st) == "eve"
 
 
+def test_unknown_grok_voice_is_refused(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setenv("DOCGEN_AI_PROVIDER", "xai")
+    cfg = _cfg(tmp_path, {})
+    st = resolve_ai_settings(cfg)
+    with pytest.raises(AIError, match="not a Grok voice"):
+        resolve_tts_voice("banana", st)
+
+
 def test_openai_client_grok_passes_base_url(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: