diff --git a/capa/features/extractors/vmray/models.py b/capa/features/extractors/vmray/models.py
index 47342b5e7..63ef3b273 100644
--- a/capa/features/extractors/vmray/models.py
+++ b/capa/features/extractors/vmray/models.py
@@ -124,8 +124,8 @@ class FunctionCall(BaseModel):
     name: CallName
     # addr: HexInt
     # from_addr: HexInt = Field(alias="from")
-    params_in: Params = Field(alias="in", default=None)
-    params_out: Params = Field(alias="out", default=None)
+    params_in: Optional[Params] = Field(alias="in", default=None)
+    params_out: Optional[Params] = Field(alias="out", default=None)
 
 
 class FunctionReturn(BaseModel):