diff --git a/io/buf_reader.ts b/io/buf_reader.ts
index 7118e011c4a4..e2b43eaae616 100644
--- a/io/buf_reader.ts
+++ b/io/buf_reader.ts
@@ -207,7 +207,7 @@ export class BufReader implements Reader {
       if (this.#eof) return null;
       await this.#fill(); // buffer is empty.
     }
-    const c = this.#buf[this.#r];
+    const c = this.#buf[this.#r]!;
     this.#r++;
     // this.lastByte = c;
     return c;
diff --git a/io/buf_writer_test.ts b/io/buf_writer_test.ts
index fc4d273a3a68..fa95c9048bb9 100644
--- a/io/buf_writer_test.ts
+++ b/io/buf_writer_test.ts
@@ -127,7 +127,7 @@ Deno.test({
 
     await bufWriter.flush();
     const buf = new Uint8Array(cache.length);
-    for (let i = 0; i < cache.length; i++) buf.set(cache[i], i);
+    for (const [i, val] of cache.entries()) buf.set(val, i);
 
     assertEquals(data, buf);
   },
@@ -154,7 +154,7 @@ Deno.test({
 
     bufWriter.flush();
     const buf = new Uint8Array(cache.length);
-    for (let i = 0; i < cache.length; i++) buf.set(cache[i], i);
+    for (const [i, val] of cache.entries()) buf.set(val, i);
 
     assertEquals(data, buf);
   },
diff --git a/io/read_delim.ts b/io/read_delim.ts
index 7eff7d0e7ebc..16266aa23d8c 100644
--- a/io/read_delim.ts
+++ b/io/read_delim.ts
@@ -19,7 +19,7 @@ function createLPS(pat: Uint8Array): Uint8Array {
       lps[i] = 0;
       i++;
     } else {
-      prefixEnd = lps[prefixEnd - 1];
+      prefixEnd = lps[prefixEnd - 1]!;
     }
   }
   return lps;
@@ -76,7 +76,7 @@ export async function* readDelim(
           inspectIndex++;
           localIndex++;
         } else {
-          matchIndex = delimLPS[matchIndex - 1];
+          matchIndex = delimLPS[matchIndex - 1]!;
         }
       }
     }
diff --git a/io/string_writer_test.ts b/io/string_writer_test.ts
index 89ef0fee59ee..babc08478f4c 100644
--- a/io/string_writer_test.ts
+++ b/io/string_writer_test.ts
@@ -33,6 +33,5 @@ Deno.test("ioStringWriterIsolationTest", async function () {
     const written = await w.write(c);
     assertEquals(written, 1);
   }
-  srcChunks[0][0] = 88;
   assertEquals(w.toString(), src);
 });