diff --git a/src/heretic/scorers/keyword_rate.py b/src/heretic/scorers/keyword_rate.py
index 9895f039..19c0e77e 100644
--- a/src/heretic/scorers/keyword_rate.py
+++ b/src/heretic/scorers/keyword_rate.py
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 # Copyright (C) 2025-2026  Philipp Emanuel Weidmann <pew@worldwidemann.com> + contributors
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 from rich.markup import escape
 
 from heretic.config import DatasetSpecification, SingleDatasetSpecification
@@ -65,6 +65,17 @@ class Settings(BaseModel):
         description="Strings whose presence in a response (case insensitive) identifies the response as a keyword match.",
     )
 
+    @field_validator("keyword_markers")
+    @classmethod
+    def validate_keyword_markers(cls, value: list[str]) -> list[str]:
+        if not value:
+            raise ValueError("keyword marker list cannot be empty")
+
+        if any(not marker.strip() for marker in value):
+            raise ValueError("keyword markers cannot be empty or whitespace")
+
+        return value
+
     print_responses: bool = Field(
         default=False,
         description="Whether to print prompt/response pairs when counting keyword matches.",
diff --git a/tests/test_keyword_rate.py b/tests/test_keyword_rate.py
index 2ef98c76..7bbb7fbf 100644
--- a/tests/test_keyword_rate.py
+++ b/tests/test_keyword_rate.py
@@ -6,6 +6,7 @@
 from typing import cast
 from unittest.mock import patch
 
+from pydantic import ValidationError
 from rich.console import Console
 
 from heretic.config import Settings as HereticSettings
@@ -23,6 +24,20 @@ def get_responses(self, prompts: list[Prompt]) -> list[str]:
 
 
 class KeywordRateTests(unittest.TestCase):
+    def test_rejects_empty_keyword_markers(self) -> None:
+        for markers in [[""], [" ", "\t"]]:
+            with self.subTest(markers=markers):
+                with self.assertRaisesRegex(
+                    ValidationError, "keyword markers cannot be empty or whitespace"
+                ):
+                    Settings(keyword_markers=markers)
+
+    def test_rejects_empty_keyword_marker_list(self) -> None:
+        with self.assertRaisesRegex(
+            ValidationError, "keyword marker list cannot be empty"
+        ):
+            Settings(keyword_markers=[])
+
     def test_print_responses_shows_brackets_literally(self) -> None:
         scorer = KeywordRate(
             heretic_settings=HereticSettings.model_construct(),