diff --git a/src/vs/base/node/encoding.ts b/src/vs/base/node/encoding.ts
index c45032131704609c0cfa04538a6b39c9dfa54110..c4cd563ef8f3ae855b458f410f2dcaf2b3a14b1f 100644
--- a/src/vs/base/node/encoding.ts
+++ b/src/vs/base/node/encoding.ts
@@ -210,6 +210,17 @@ async function guessEncodingByBuffer(buffer: Buffer): Promise<string | null> {
 		return null;
 	}
 
+	// Ignore 'ascii' as guessed encoding because that
+	// is almost never what we want, rather fallback
+	// to the configured encoding then. Otherwise,
+	// opening a ascii-only file with auto guessing
+	// enabled will put the file into 'ascii' mode
+	// and thus typing any special characters is
+	// not possible anymore.
+	if (guessed.encoding.toLowerCase() === 'ascii') {
+		return null;
+	}
+
 	return toIconvLiteEncoding(guessed.encoding);
 }
 
diff --git a/src/vs/base/test/node/encoding/encoding.test.ts b/src/vs/base/test/node/encoding/encoding.test.ts
index d8730a52b366ac915e3dfa4767ad1134b7ef96d7..eef17fbddb61a26767f1f52706425538eadafbe9 100644
--- a/src/vs/base/test/node/encoding/encoding.test.ts
+++ b/src/vs/base/test/node/encoding/encoding.test.ts
@@ -200,7 +200,7 @@ suite('Encoding', () => {
 		const file = getPathFromAmdModule(require, './fixtures/some_ansi.css');
 		const buffer = await readExactlyByFile(file, 512 * 8);
 		const mimes = await encoding.detectEncodingFromBuffer(buffer, true);
-		assert.equal(mimes.encoding, 'ascii');
+		assert.equal(mimes.encoding, null);
 	});
 
 	test('autoGuessEncoding (ShiftJIS)', async function () {