diff --git a/crates/polars-core/src/series/ops/null.rs b/crates/polars-core/src/series/ops/null.rs
index b5b8b4ad6362..40ef6a383dd6 100644
--- a/crates/polars-core/src/series/ops/null.rs
+++ b/crates/polars-core/src/series/ops/null.rs
@@ -89,7 +89,7 @@ impl Series {
             },
             DataType::Null => Series::new_null(name, size),
             DataType::Unknown(kind) => {
-                let dtype = kind.materialize().expect("expected known type");
+                let dtype = kind.materialize().unwrap_or(DataType::Null);
                 Series::full_null(name, size, &dtype)
             },
             #[cfg(feature = "object")]