diff --git a/packages/fresh/src/dev/dev_build_cache.ts b/packages/fresh/src/dev/dev_build_cache.ts
index 3678d0ffba4..559e463e488 100644
--- a/packages/fresh/src/dev/dev_build_cache.ts
+++ b/packages/fresh/src/dev/dev_build_cache.ts
@@ -296,9 +296,8 @@ export class DiskBuildCache<State> implements DevBuildCache<State> {
     await Deno.writeFile(filePath, content);
   }
 
-  // deno-lint-ignore require-await
-  async readFile(_pathname: string): Promise<StaticFile | null> {
-    throw new Error("Not implemented in build mode");
+  readFile(_pathname: string): Promise<StaticFile | null> {
+    return Promise.reject(new Error("Not implemented in build mode"));
   }
 
   async prepare(): Promise<void> {
diff --git a/packages/fresh/src/router.ts b/packages/fresh/src/router.ts
index c0bbfa9293a..f750c856dc0 100644
--- a/packages/fresh/src/router.ts
+++ b/packages/fresh/src/router.ts
@@ -174,7 +174,15 @@ export class UrlPatternRouter<T> implements Router<T> {
 
         // Decode matched params
         for (const [key, value] of Object.entries(match.pathname.groups)) {
-          result.params[key] = value === undefined ? "" : decodeURI(value);
+          if (value === undefined) {
+            result.params[key] = "";
+          } else {
+            try {
+              result.params[key] = decodeURI(value);
+            } catch {
+              result.params[key] = value;
+            }
+          }
         }
       }