diff --git a/lib/cache.ts b/lib/cache.ts
index 605bae68..4b0de14a 100644
--- a/lib/cache.ts
+++ b/lib/cache.ts
@@ -21,7 +21,7 @@ export = class Cache<T> {
     this.cache.delete(id);
   }
 
-  apply(id: string, value) {
+  apply(id: string, value): T {
     if (this.has(id)) return this.get(id);
 
     if (typeof value === 'function') value = value();
diff --git a/lib/html_tag.ts b/lib/html_tag.ts
index c5efc318..4d357a70 100644
--- a/lib/html_tag.ts
+++ b/lib/html_tag.ts
@@ -15,7 +15,7 @@ function encSrcset(str: string) {
 }
 
 function htmlTag(tag: string, attrs: {
-  [key: string]: string | boolean;
+  [key: string]: string | boolean | null | undefined;
 }, text?: string, escape = true) {
   if (!tag) throw new TypeError('tag is required!');
 
diff --git a/lib/permalink.ts b/lib/permalink.ts
index 6b999b57..53fdac89 100644
--- a/lib/permalink.ts
+++ b/lib/permalink.ts
@@ -2,14 +2,19 @@ import escapeRegExp from './escape_regexp';
 
 const rParam = /:(\w*[^_\W])/g;
 
+interface Options {
+  segments?: {
+    [key: string]: RegExp | string;
+  }
+}
+
 class Permalink {
   rule: string;
   regex: RegExp;
   params: string[];
 
-  constructor(rule: string, options: { segments?: any; }) {
+  constructor(rule: string, options: Options = {}) {
     if (!rule) { throw new TypeError('rule is required!'); }
-    options = options || {};
     const segments = options.segments || {};
     const params = [];
     const regex = escapeRegExp(rule)
diff --git a/lib/relative_url.ts b/lib/relative_url.ts
index 8c2080a2..a90fab00 100644
--- a/lib/relative_url.ts
+++ b/lib/relative_url.ts
@@ -1,6 +1,6 @@
 import encodeURL from './encode_url';
 import Cache from './cache';
-const cache = new Cache();
+const cache = new Cache<string>();
 
 function relativeUrlHelper(from = '', to = '') {
   return cache.apply(`${from}-${to}`, () => {
diff --git a/lib/url_for.ts b/lib/url_for.ts
index 482b8e53..7ab50408 100644
--- a/lib/url_for.ts
+++ b/lib/url_for.ts
@@ -3,7 +3,7 @@ import encodeURL from './encode_url';
 import relative_url from './relative_url';
 import prettyUrls from './pretty_urls';
 import Cache from './cache';
-const cache = new Cache();
+const cache = new Cache<string>();
 
 function urlForHelper(path = '/', options) {
   if (/^(#|\/\/|http(s)?:)/.test(path)) return path;