diff --git a/noko_client/schemas/entries_parameters.py b/noko_client/schemas/entries_parameters.py
index f80e284..5c44a5b 100644
--- a/noko_client/schemas/entries_parameters.py
+++ b/noko_client/schemas/entries_parameters.py
@@ -60,7 +60,7 @@ class GetNokoEntriesParameters(BaseModel):
 
     user_ids: str | list | None = None
     description: str | None = None
-    project_ids: str | list | None = None
+    project_ids: str | int | list | None = None
     tag_ids: str | list | None = None
     tag_filter_type: str | None = None
     from_: str | datetime | None = Field(alias="from", default=None)
diff --git a/noko_client/schemas/validators.py b/noko_client/schemas/validators.py
index 85a1bb5..b6b9474 100644
--- a/noko_client/schemas/validators.py
+++ b/noko_client/schemas/validators.py
@@ -26,6 +26,8 @@ def format_date(value: str | datetime) -> str:
 
 def format_id_lists(value: str | int | list | None) -> str | None:
     """If IDs provided as lists, convert to a comma separated string."""
+    if value is None:
+        return None
     return list_to_string(value) if isinstance(value, list) else str(value)