diff --git a/prototype/Doubt/Patch.swift b/prototype/Doubt/Patch.swift
index 524adb146..c3a67e65f 100644
--- a/prototype/Doubt/Patch.swift
+++ b/prototype/Doubt/Patch.swift
@@ -74,7 +74,14 @@ extension Patch {
 extension Patch {
 	/// Returns a function which computes the size of a `patch` as the sum of the sizes of its terms, as computed by `size`.
 	public static func sum(@noescape size: A throws -> Int)(_ patch: Patch) rethrows -> Int {
-		return try (patch.state.before.map(size) ?? 0) + (patch.state.after.map(size) ?? 0)
+		switch patch {
+		case let .Replace(a, b):
+			return try size(a) + size(b)
+		case let .Insert(b):
+			return try size(b)
+		case let .Delete(a):
+			return try size(a)
+		}
 	}
 
 	/// Returns a function which computes the size of a `patch` as the absolute difference of the sizes of its terms, as computed by `size`.