diff --git a/prototype/Doubt/Matrix.swift b/prototype/Doubt/Matrix.swift
index e56bb7d5c..9b109f6c5 100644
--- a/prototype/Doubt/Matrix.swift
+++ b/prototype/Doubt/Matrix.swift
@@ -20,7 +20,7 @@ struct Matrix<A> {
 	let width: Int
 	let height: Int
 
-	let values: [Memo<A>]
+	private let values: [Memo<A>]
 
 	subscript (i: Int, j: Int) -> Memo<A>? {
 		guard i < width && j < height else { return nil }