diff --git a/src/longpath/cli.py b/src/longpath/cli.py
index 6229fb5..73e587e 100644
--- a/src/longpath/cli.py
+++ b/src/longpath/cli.py
@@ -233,7 +233,7 @@ def _policy_lines(pal: Palette) -> List[str]:
 
 
 def _clean_base(base: Optional[str]) -> Optional[str]:
-    # `--base "C:\dest\"` in PowerShell/cmd turns the trailing \" into a
+    # `--base "C:\\dest\\"` in PowerShell/cmd turns the trailing \\" into a
     # literal quote; a path cannot legally end in one, so strip it.
     if base is None:
         return None
@@ -247,6 +247,9 @@ def _run_scan(args: argparse.Namespace) -> int:
     if args.limit < 16:
         _err("longpath: --limit must be at least 16")
         return EXIT_ERROR
+    if args.top < 0:
+        _err("longpath: --top must be >= 0")
+        return EXIT_ERROR
 
     base = _clean_base(args.base)
     result = scan_tree(args.dir, limit=args.limit, base=base, exclude=args.exclude)
@@ -472,7 +475,7 @@ def main(argv: Optional[List[str]] = None) -> int:
         except (AttributeError, OSError):
             pass
 
-    # sugar: `longpath D:\dir` == `longpath scan D:\dir`; bare `longpath` scans .
+    # sugar: `longpath D:\\dir` == `longpath scan D:\\dir`; bare `longpath` scans .
     if not argv:
         argv = ["scan"]
     elif argv[0] not in _SUBCOMMANDS and not argv[0].startswith("-"):
diff --git a/src/longpath/validate.py b/src/longpath/validate.py
new file mode 100644
index 0000000..8796fe0
--- /dev/null
+++ b/src/longpath/validate.py
@@ -0,0 +1,7 @@
+"""Small CLI argument guards."""
+
+
+def require_nonneg_top(n: int) -> int:
+    if int(n) < 0:
+        raise ValueError("--top must be >= 0")
+    return int(n)
diff --git a/tests/test_negative_top.py b/tests/test_negative_top.py
new file mode 100644
index 0000000..29770f1
--- /dev/null
+++ b/tests/test_negative_top.py
@@ -0,0 +1,18 @@
+import pytest
+
+from longpath.validate import require_nonneg_top
+
+from conftest import run_cli
+
+
+def test_require_nonneg_top():
+    assert require_nonneg_top(0) == 0
+    assert require_nonneg_top(20) == 20
+    with pytest.raises(ValueError, match="--top"):
+        require_nonneg_top(-1)
+
+
+def test_scan_negative_top_exit_2(tree):
+    p = run_cli("scan", str(tree), "--top", "-1")
+    assert p.returncode == 2
+    assert "--top must be >= 0" in p.stderr