diff --git a/package.json b/package.json
index 7c75a2a..2498227 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,9 @@
   "private": true,
   "scripts": {
     "start": "parcel index.html",
-    "build": "parcel build index.html --no-source-maps"
+    "build": "parcel build index.html --no-source-maps",
+    "prebuild": "yarn run typecheck",
+    "typecheck": "tsc --noEmit src/saimiar_morphology.ts"
   },
   "devDependencies": {
     "@parcel/transformer-image": "2.0.0-rc.0",
diff --git a/src/saimiar_morphology.ts b/src/saimiar_morphology.ts
index e8cde92..1083a82 100644
--- a/src/saimiar_morphology.ts
+++ b/src/saimiar_morphology.ts
@@ -26,7 +26,7 @@ function declineSaimiar(entry) {
 	return null;
 }
 
-function vowelDeclension(sai) {
+function vowelDeclension(sai: string) {
 	const {root, ending} = rootEndingPair(sai);
 	const adpEnding = ending === 'u' ? 'ys' : `${ending}s`;