diff --git a/src/mol-data/db/column.ts b/src/mol-data/db/column.ts
index b43bd452ef2aae23ce95b06a01fb9a18489f7599..33672a0e525f66fe1c3bcd430e70698a4c10d9f0 100644
--- a/src/mol-data/db/column.ts
+++ b/src/mol-data/db/column.ts
@@ -302,7 +302,7 @@ function arrayColumn<T extends Column.Schema>({ array, schema, valueKind }: Colu
     }
 }
 
-function windowColumn<T>(column: Column<T>, start: number, end: number) {
+function windowColumn<T>(column: Column<T>, start: number, end: number): Column<T> {
     if (!column.isDefined) return Column.Undefined(end - start, column.schema);
     if (start === 0 && end === column.rowCount) return column;
     if (!!column.__array && ColumnHelpers.isTypedArray(column.__array)) return windowTyped(column, start, end);