diff --git a/fj-app/src/main.rs b/fj-app/src/main.rs
index 6589f1553..39c79a531 100644
--- a/fj-app/src/main.rs
+++ b/fj-app/src/main.rs
@@ -8,7 +8,7 @@ mod window;
 use std::path::PathBuf;
 use std::{collections::HashMap, time::Instant};
 
-use fj_host::Model;
+use fj_host::{Model, Parameters};
 use fj_interop::{debug::DebugInfo, mesh::Mesh};
 use fj_kernel::algorithms::{triangulate, Tolerance};
 use fj_math::{Aabb, Point, Scalar};
@@ -83,7 +83,7 @@ fn main() -> anyhow::Result<()> {
     };
 
     if let Some(path) = args.export {
-        let shape = model.load_once(&parameters)?;
+        let shape = model.load_once(&Parameters(parameters))?;
         let shape = shape_processor.process(&shape);
 
         let vertices =
@@ -111,7 +111,7 @@ fn main() -> anyhow::Result<()> {
         return Ok(());
     }
 
-    let watcher = model.load_and_watch(parameters)?;
+    let watcher = model.load_and_watch(Parameters(parameters))?;
 
     let event_loop = EventLoop::new();
     let window = Window::new(&event_loop);
diff --git a/fj-host/src/lib.rs b/fj-host/src/lib.rs
index 9689a68cb..64ba006e7 100644
--- a/fj-host/src/lib.rs
+++ b/fj-host/src/lib.rs
@@ -74,7 +74,7 @@ impl Model {
     /// model for changes, reloading it continually.
     pub fn load_once(
         &self,
-        arguments: &HashMap<String, String>,
+        arguments: &Parameters,
     ) -> Result<fj::Shape, Error> {
         let manifest_path = self.manifest_path.display().to_string();
 
@@ -120,7 +120,7 @@ impl Model {
     /// be queried for changes to the model.
     pub fn load_and_watch(
         self,
-        parameters: HashMap<String, String>,
+        parameters: Parameters,
     ) -> Result<Watcher, Error> {
         let (tx, rx) = mpsc::sync_channel(0);
         let tx2 = tx.clone();
@@ -202,7 +202,7 @@ pub struct Watcher {
     _watcher: Box<dyn notify::Watcher>,
     channel: mpsc::Receiver<()>,
     model: Model,
-    parameters: HashMap<String, String>,
+    parameters: Parameters,
 }
 
 impl Watcher {
@@ -243,6 +243,16 @@ impl Watcher {
     }
 }
 
+/// Parameters that are passed to a model
+pub struct Parameters(pub HashMap<String, String>);
+
+impl Parameters {
+    /// Construct an empty instance of `Parameters`
+    pub fn empty() -> Self {
+        Self(HashMap::new())
+    }
+}
+
 /// An error that can occur when loading or reloading a model
 #[derive(Debug, Error)]
 pub enum Error {
@@ -263,5 +273,4 @@ pub enum Error {
     Notify(#[from] notify::Error),
 }
 
-type ModelFn =
-    unsafe extern "C" fn(args: &HashMap<String, String>) -> fj::Shape;
+type ModelFn = unsafe extern "C" fn(args: &Parameters) -> fj::Shape;