diff --git a/lib/RawSource.js b/lib/RawSource.js
index 71f1341..098d317 100644
--- a/lib/RawSource.js
+++ b/lib/RawSource.js
@@ -13,7 +13,7 @@ class RawSource extends Source {
 		super();
 		const isBuffer = Buffer.isBuffer(value);
 		if (!isBuffer && typeof value !== "string") {
-			throw new TypeError("argument 'value' must be either string of Buffer");
+			throw new TypeError("argument 'value' must be either string or Buffer");
 		}
 		this._valueIsBuffer = !convertToString && isBuffer;
 		this._value = convertToString && isBuffer ? undefined : value;