diff --git a/lib/models/os.ts b/lib/models/os.ts
index a963db235..1b772e05b 100644
--- a/lib/models/os.ts
+++ b/lib/models/os.ts
@@ -472,7 +472,7 @@ const getOsModel = function (
 				url: `/device-types/v1/${deviceType}/images/${version}/download-size`,
 				baseUrl: apiUrl,
 			});
-			return body.size;
+			return body.size as number;
 		},
 	);
 
@@ -937,8 +937,14 @@ const getOsModel = function (
 	};
 
 	return {
-		_getNormalizedDeviceTypeSlug,
-		_getDownloadSize,
+		// Cast the exported types for internal methods so `@types/memoizee` can be a dev depenency.
+		_getNormalizedDeviceTypeSlug: _getNormalizedDeviceTypeSlug as (
+			deviceTypeSlug: string,
+		) => Promise<string>,
+		_getDownloadSize: _getDownloadSize as (
+			deviceType: string,
+			version: string,
+		) => Promise<number>,
 		_clearDeviceTypesAndOsVersionCaches,
 		_getMaxSatisfyingVersion,
 		OsTypes,
diff --git a/package.json b/package.json
index 7b329d546..414a97bc3 100644
--- a/package.json
+++ b/package.json
@@ -62,6 +62,8 @@
     "@balena/lint": "^6.1.1",
     "@types/chai": "^4.3.0",
     "@types/chai-as-promised": "^7.1.4",
+    "@types/lodash": "^4.14.182",
+    "@types/memoizee": "^0.4.7",
     "@types/mocha": "^5.2.7",
     "@types/ndjson": "^2.0.0",
     "@types/sinon": "^10.0.6",
@@ -103,8 +105,6 @@
   "dependencies": {
     "@balena/es-version": "^1.0.0",
     "@types/json-schema": "^7.0.9",
-    "@types/lodash": "^4.14.168",
-    "@types/memoizee": "^0.4.5",
     "@types/node": "^10.17.55",
     "abortcontroller-polyfill": "^1.7.1",
     "balena-auth": "^4.1.0",