diff --git a/src/is_package_json_version_upgraded.ts b/src/is_package_json_version_upgraded.ts
index 8400bf6..cc45b00 100644
--- a/src/is_package_json_version_upgraded.ts
+++ b/src/is_package_json_version_upgraded.ts
@@ -37,7 +37,7 @@ export async function action(
     const to_version = await getPackageJsonVersion({ owner, repo, branch });
 
     if( to_version === undefined ){
-        throw new Error("No version in package.json on ${owner}/${repo}#${branch} (or repo is private)");
+        throw new Error(`No version in package.json on ${owner}/${repo}#${branch} (or repo is private)`);
     }
 
     core.debug(`Version on ${owner}/${repo}#${branch} is ${NpmModuleVersion.stringify(to_version)}`);