diff --git a/polars_business/polars_business/polars_business/__init__.py b/polars_business/polars_business/polars_business/__init__.py
index 73464be..84acf2f 100644
--- a/polars_business/polars_business/polars_business/__init__.py
+++ b/polars_business/polars_business/polars_business/__init__.py
@@ -4,11 +4,20 @@
 from polars.utils.udfs import _get_shared_lib_location
 import re
 from datetime import date
+import sys
 
 from polars_business.ranges import date_range
 
 from polars.type_aliases import PolarsDataType
-from typing import Sequence, cast, Iterable, Protocol
+from typing import Iterable, Literal, Protocol, Sequence, cast, get_args
+
+if sys.version_info >= (3, 10):
+    from typing import TypeAlias
+else:
+    from typing_extensions import TypeAlias
+
+RollStrategy: TypeAlias = Literal["raise", "forward", "backward"]
+
 
 lib = _get_shared_lib_location(__file__)
 
@@ -81,7 +90,7 @@ def offset_by(
         *,
         weekend: Sequence[str] = ("Sat", "Sun"),
         holidays: Sequence[date] | None = None,
-        roll: str = "raise",
+        roll: RollStrategy = "raise",
     ) -> BExpr:
         """
         Offset this date by a relative time offset.
@@ -173,6 +182,12 @@ def offset_by(
         │ 2024-01-04 ┆ -3bd ┆ 2024-01-01   │
         └────────────┴──────┴──────────────┘
         """
+        if roll not in (valid_roll_strategies := get_args(RollStrategy)):
+            allowed = ", ".join(repr(m) for m in valid_roll_strategies)
+            raise ValueError(
+                f"`roll` strategy must be one of {{{allowed}}}, got {roll!r}"
+            )
+
         if (
             isinstance(by, str)
             and (match := re.search(r"(\d+bd)", by)) is not None
diff --git a/polars_business/polars_business/src/business_days.rs b/polars_business/polars_business/src/business_days.rs
index aae3054..a350ee5 100644
--- a/polars_business/polars_business/src/business_days.rs
+++ b/polars_business/polars_business/src/business_days.rs
@@ -25,7 +25,9 @@ pub(crate) fn calculate_advance(
                 let date = NaiveDateTime::from_timestamp_opt(date as i64 * 24 * 60 * 60, 0)
                     .unwrap()
                     .format("%Y-%m-%d");
-                polars_bail!(ComputeError: format!("date {} is not a business date, cannot advance. `roll` argument coming soon.", date))
+                polars_bail!(ComputeError:
+                    format!("date {} is not a business date, cannot advance; set a valid `roll` strategy.", date)
+                )
             };
         }
         "forward" => {
@@ -50,7 +52,11 @@ pub(crate) fn calculate_advance(
                 }
             }
         }
-        _ => unreachable!(),
+        _ => {
+            polars_bail!(InvalidOperation:
+                "`roll` must be one of 'raise', 'forward' or 'backward'; found '{}'", roll
+            )
+        }
     }
 
     if offset > 0 {
diff --git a/polars_business/tests/test_business_offsets.py b/polars_business/tests/test_business_offsets.py
index 446fe93..8e18289 100644
--- a/polars_business/tests/test_business_offsets.py
+++ b/polars_business/tests/test_business_offsets.py
@@ -220,3 +220,11 @@ def test_within_group_by() -> None:
         }
     )
     assert_frame_equal(result, expected)
+
+
+def test_invalid_roll_strategy() -> None:
+    df = pl.DataFrame(
+        {"date": pl.date_range(dt.date(2023, 12, 1), dt.date(2023, 12, 5), eager=True)}
+    )
+    with pytest.raises(ValueError):
+        df.with_columns(plb.col("date").bdt.offset_by("1bd", roll="cabbage"))  # type: ignore[arg-type]