Turns out I was wrong, the @size helper attempts to cast to a number regardless of the input and it uses that. Fixed the test.

src/bin.rs

@@ -353,6 +353,10 @@ impl Castable for serde_json::Value {
                 })
                 .ok(),
             (serde_json::Value::Number(_), "number") => Some(IceResult::from_borrowed(self)),
+            (serde_json::Value::Null, "number") => None,
+            (serde_json::Value::Bool(_), "number") => None,
+            (serde_json::Value::Array(_), "number") => None,
+            (serde_json::Value::Object(_), "number") => None,
             (_, _) => panic!("Unimplemented cast"),
         }
     }