diff --git a/narwhals/_arrow/utils.py b/narwhals/_arrow/utils.py
index 6f7517aeb..b8294839c 100644
--- a/narwhals/_arrow/utils.py
+++ b/narwhals/_arrow/utils.py
@@ -51,7 +51,7 @@ def translate_dtype(dtype: Any) -> dtypes.DType:
         return dtypes.Duration()
     if pa.types.is_dictionary(dtype):
         return dtypes.Categorical()
-    raise AssertionError
+    return dtypes.Unknown()  # pragma: no cover
 
 
 def narwhals_to_native_dtype(dtype: dtypes.DType | type[dtypes.DType]) -> Any:
diff --git a/narwhals/_duckdb/dataframe.py b/narwhals/_duckdb/dataframe.py
index b7b679208..8de244658 100644
--- a/narwhals/_duckdb/dataframe.py
+++ b/narwhals/_duckdb/dataframe.py
@@ -42,12 +42,7 @@ def map_duckdb_dtype_to_narwhals_dtype(
         return dtypes.Boolean()
     if duckdb_dtype == "INTERVAL":
         return dtypes.Duration()
-    msg = (  # pragma: no cover
-        f"Invalid dtype, got: {duckdb_dtype}.\n\n"
-        "If you believe this dtype should be supported in Narwhals, "
-        "please report an issue at https://github.com/narwhals-dev/narwhals."
-    )
-    raise AssertionError(msg)
+    return dtypes.Unknown()
 
 
 class DuckDBInterchangeFrame:
diff --git a/narwhals/_ibis/dataframe.py b/narwhals/_ibis/dataframe.py
index 2e7bb0a1d..e2baa4ec4 100644
--- a/narwhals/_ibis/dataframe.py
+++ b/narwhals/_ibis/dataframe.py
@@ -40,12 +40,7 @@ def map_ibis_dtype_to_narwhals_dtype(
         return dtypes.Date()
     if ibis_dtype.is_timestamp():
         return dtypes.Datetime()
-    msg = (  # pragma: no cover
-        f"Invalid dtype, got: {ibis_dtype}.\n\n"
-        "If you believe this dtype should be supported in Narwhals, "
-        "please report an issue at https://github.com/narwhals-dev/narwhals."
-    )
-    raise AssertionError(msg)
+    return dtypes.Unknown()  # pragma: no cover
 
 
 class IbisInterchangeFrame:
diff --git a/tests/frame/interchange_schema_test.py b/tests/frame/interchange_schema_test.py
index 56289dfd7..a73ca6259 100644
--- a/tests/frame/interchange_schema_test.py
+++ b/tests/frame/interchange_schema_test.py
@@ -3,6 +3,7 @@
 from datetime import timedelta
 
 import duckdb
+import pandas as pd
 import polars as pl
 import pytest
 
@@ -211,3 +212,10 @@ def test_get_level() -> None:
         nw.get_level(nw.from_native(df.__dataframe__(), eager_or_interchange_only=True))
         == "interchange"
     )
+
+
+def test_unknown_dtype() -> None:
+    df = pd.DataFrame({"a": [1, 2, 3]})
+    rel = duckdb.from_df(df).select("cast(a as int128) as a")
+    result = nw.from_native(rel).schema
+    assert result == {"a": nw.Unknown}