diff --git a/src/semchunk/semchunk.py b/src/semchunk/semchunk.py
index 6538383..a8cb1e2 100644
--- a/src/semchunk/semchunk.py
+++ b/src/semchunk/semchunk.py
@@ -227,6 +227,10 @@ def chunk(
     ilgs_doc = None
 
     if is_first_call := not _recursion_depth:
+        # A non-positive chunk_size fits no token, so the recursion never terminates.
+        if chunk_size < 1:
+            raise ValueError(f"chunk_size must be a positive integer, not {chunk_size!r}.")
+
         # Memoize the token counter if desired.
         if memoize:
             token_counter = _memoized_token_counters.setdefault(token_counter, lru_cache(cache_maxsize)(token_counter))
diff --git a/tests/test_semchunk.py b/tests/test_semchunk.py
index 209de8f..ba6aad6 100644
--- a/tests/test_semchunk.py
+++ b/tests/test_semchunk.py
@@ -199,5 +199,27 @@ def test_semchunk() -> None:
     # Test chunking whitespace to ensure no errors are raised.
     semchunk.chunk('\n\n', 512, lambda *args: 0)
 
+def test_non_positive_chunk_size() -> None:
+    """Test that a non-positive chunk size raises a clear ``ValueError`` rather than recursing until a ``RecursionError``."""
+    token_counter = len
+
+    for bad_size in (0, -1, -100):
+        # Directly via `chunk`.
+        try:
+            semchunk.chunk('hello world', bad_size, token_counter)
+        except ValueError:
+            pass
+        else:
+            raise AssertionError(f'chunk() with chunk_size={bad_size!r} did not raise ValueError')
+
+        # And via a `chunkerify`-built chunker, which defers to `chunk`.
+        try:
+            semchunk.chunkerify(token_counter, bad_size)('hello world')
+        except ValueError:
+            pass
+        else:
+            raise AssertionError(f'chunkerify() with chunk_size={bad_size!r} did not raise ValueError')
+
 if __name__ == '__main__':
-    test_semchunk()
\ No newline at end of file
+    test_semchunk()
+    test_non_positive_chunk_size()
\ No newline at end of file