diff --git a/src/dagster_pyiceberg/_db_io_manager/utils.py b/src/dagster_pyiceberg/_db_io_manager/utils.py
index 48bac48..f4c2563 100644
--- a/src/dagster_pyiceberg/_db_io_manager/utils.py
+++ b/src/dagster_pyiceberg/_db_io_manager/utils.py
@@ -115,7 +115,7 @@ def generate_multi_partitions_dimension(
                 partitions.append(partition_key)
 
         partition_expr_str = partition_expr.get(part.name)
-        if partition_expr is None:
+        if partition_expr_str is None:
             raise ValueError(
                 f"Asset '{asset_key}' has partition {part.name}, but the"
                 f" 'partition_expr' metadata does not contain a {part.name} entry,"