diff --git a/slugify/slugify.py b/slugify/slugify.py
index 27df54b..0bc9fe9 100644
--- a/slugify/slugify.py
+++ b/slugify/slugify.py
@@ -142,6 +142,8 @@ def _modern_slugify(
         raise TypeError(f"max_length must be an int, not {type(max_length).__name__}")
     if not isinstance(separator, str):
         raise TypeError(f"separator must be str, not {type(separator).__name__}")
+    if regex_pattern is not None and not isinstance(regex_pattern, (str, re.Pattern)):
+        raise TypeError(f"regex_pattern must be str, re.Pattern or None, not {type(regex_pattern).__name__}")
     if not isinstance(text, str):
         if not isinstance(text, (bytes, bytearray)):
             raise TypeError(f'text must be str, bytes or bytearray, not {type(text).__name__}')
diff --git a/tests/test_modern_unicode.py b/tests/test_modern_unicode.py
new file mode 100644
index 0000000..9aff03c
--- /dev/null
+++ b/tests/test_modern_unicode.py
@@ -0,0 +1,23 @@
+"""Modern algorithm up-front argument type validation.
+
+The modern algorithm validates caller-supplied arguments at the top of
+``_modern_slugify`` so a misuse fails with a clear, argument-specific message
+instead of a cryptic error raised deep in the pipeline. These tests cover the
+``regex_pattern`` argument alongside the existing ``max_length``/``separator``
+guards.
+"""
+import re
+
+import pytest
+
+from slugify import slugify
+
+
+def test_regex_pattern_type_is_validated_up_front():
+    with pytest.raises(TypeError, match='regex_pattern'):
+        slugify('abc', algorithm='modern', regex_pattern=123)
+
+
+def test_regex_pattern_accepts_str_and_compiled_pattern():
+    assert slugify('a b', algorithm='modern', regex_pattern=r'[^-a-z]+') == 'a-b'
+    assert slugify('a b', algorithm='modern', regex_pattern=re.compile(r'[^-a-z]+')) == 'a-b'