diff --git a/media_types/_util.ts b/media_types/_util.ts
index 114d9be40abf..34dc8e9becca 100644
--- a/media_types/_util.ts
+++ b/media_types/_util.ts
@@ -40,8 +40,9 @@ export function consumeValue(v: string): [value: string, rest: string] {
     if (r === `"`) {
       return [value, v.slice(i + 1)];
     }
-    if (r === "\\" && i + 1 < v.length && isTSpecial(v[i + 1])) {
-      value += v[i + 1];
+    const next = v[i + 1];
+    if (r === "\\" && typeof next === "string" && isTSpecial(next)) {
+      value += next;
       i++;
       continue;
     }
@@ -83,14 +84,15 @@ export function decode2331Encoding(v: string): string | undefined {
   if (sv.length !== 3) {
     return undefined;
   }
-  const charset = sv[0].toLowerCase();
+  const [sv0, , sv2] = sv as [string, string, string];
+  const charset = sv0.toLowerCase();
   if (!charset) {
     return undefined;
   }
   if (charset !== "us-ascii" && charset !== "utf-8") {
     return undefined;
   }
-  const encv = decodeURI(sv[2]);
+  const encv = decodeURI(sv2);
   if (!encv) {
     return undefined;
   }
@@ -133,7 +135,7 @@ function isTokenChar(r: string): boolean {
 }
 
 function isTSpecial(r: string): boolean {
-  return `()<>@,;:\\"/[]?=`.includes(r[0]);
+  return r[0] ? `()<>@,;:\\"/[]?=`.includes(r[0]) : false;
 }
 
 const CHAR_CODE_SPACE = " ".charCodeAt(0);
diff --git a/media_types/format_media_type.ts b/media_types/format_media_type.ts
index ad5412967afc..d277913d8ea0 100644
--- a/media_types/format_media_type.ts
+++ b/media_types/format_media_type.ts
@@ -23,7 +23,7 @@ export function formatMediaType(
   param?: Record<string, string> | Iterable<[string, string]>,
 ): string {
   let b = "";
-  const [major, sub] = type.split("/");
+  const [major = "", sub] = type.split("/");
   if (!sub) {
     if (!isToken(type)) {
       return "";
@@ -45,7 +45,7 @@ export function formatMediaType(
       if (!isToken(attribute)) {
         return "";
       }
-      const value = param[attribute];
+      const value = param[attribute]!;
       b += `; ${attribute.toLowerCase()}`;
 
       const needEnc = needsEncoding(value);
diff --git a/media_types/parse_media_type.ts b/media_types/parse_media_type.ts
index ce50be78b0fd..0f831beab17e 100644
--- a/media_types/parse_media_type.ts
+++ b/media_types/parse_media_type.ts
@@ -42,7 +42,7 @@ import { consumeMediaParam, decode2331Encoding } from "./_util.ts";
 export function parseMediaType(
   v: string,
 ): [mediaType: string, params: Record<string, string> | undefined] {
-  const [base] = v.split(";");
+  const [base] = v.split(";") as [string];
   const mediaType = base.toLowerCase().trim();
 
   const params: Record<string, string> = {};