diff --git a/fj-app/src/args.rs b/fj-app/src/args.rs
index 16cada701..aabaa3e60 100644
--- a/fj-app/src/args.rs
+++ b/fj-app/src/args.rs
@@ -1,5 +1,7 @@
 use std::{path::PathBuf, str::FromStr as _};
 
+use anyhow::anyhow;
+use fj_host::Parameters;
 use fj_kernel::algorithms::Tolerance;
 use fj_math::Scalar;
 
@@ -15,8 +17,8 @@ pub struct Args {
     pub export: Option<PathBuf>,
 
     /// Parameters for the model, each in the form `key=value`
-    #[clap(short, long)]
-    pub parameters: Vec<String>,
+    #[clap(short, long, parse(try_from_str = parse_parameters))]
+    pub parameters: Option<Parameters>,
 
     /// Model deviation tolerance
     #[clap[short, long, parse(try_from_str = parse_tolerance)]]
@@ -33,6 +35,27 @@ impl Args {
     }
 }
 
+fn parse_parameters(input: &str) -> anyhow::Result<Parameters> {
+    let mut parameters = Parameters::empty();
+
+    for parameter in input.split(',') {
+        let mut parameter = parameter.splitn(2, '=');
+
+        let key = parameter
+            .next()
+            .ok_or_else(|| anyhow!("Expected model parameter key"))?
+            .to_owned();
+        let value = parameter
+            .next()
+            .ok_or_else(|| anyhow!("Expected model parameter value"))?
+            .to_owned();
+
+        parameters.0.insert(key, value);
+    }
+
+    Ok(parameters)
+}
+
 fn parse_tolerance(input: &str) -> anyhow::Result<Tolerance> {
     let tolerance = f64::from_str(input)?;
     let tolerance = Scalar::from_f64(tolerance);
diff --git a/fj-app/src/main.rs b/fj-app/src/main.rs
index 6589f1553..b44061526 100644
--- a/fj-app/src/main.rs
+++ b/fj-app/src/main.rs
@@ -6,9 +6,10 @@ mod input;
 mod window;
 
 use std::path::PathBuf;
-use std::{collections::HashMap, time::Instant};
+use std::time::Instant;
 
-use fj_host::Model;
+use anyhow::anyhow;
+use fj_host::{Model, Parameters};
 use fj_interop::{debug::DebugInfo, mesh::Mesh};
 use fj_kernel::algorithms::{triangulate, Tolerance};
 use fj_math::{Aabb, Point, Scalar};
@@ -48,35 +49,16 @@ fn main() -> anyhow::Result<()> {
     let config = Config::load()?;
 
     let mut path = config.default_path.unwrap_or_else(|| PathBuf::from(""));
-    match args.model.or(config.default_model) {
-        Some(model) => {
-            path.push(model);
-        }
-        None => {
-            anyhow::bail!(
-                "No model specified, and no default model configured.\n\
+    let model = args.model.or(config.default_model).ok_or_else(|| {
+        anyhow!(
+            "No model specified, and no default model configured.\n\
                 Specify a model by passing `--model path/to/model`."
-            );
-        }
-    }
+        )
+    })?;
+    path.push(model);
 
     let model = Model::from_path(path, config.target_dir)?;
-
-    let mut parameters = HashMap::new();
-    for parameter in args.parameters {
-        let mut parameter = parameter.splitn(2, '=');
-
-        let key = parameter
-            .next()
-            .expect("model parameter: key not found")
-            .to_owned();
-        let value = parameter
-            .next()
-            .expect("model parameter: value not found")
-            .to_owned();
-
-        parameters.insert(key, value);
-    }
+    let parameters = args.parameters.unwrap_or_else(Parameters::empty);
 
     let shape_processor = ShapeProcessor {
         tolerance: args.tolerance,
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;