diff --git a/src/Whim.Tests/Store/WorkspaceSector/WorkspacePickersTests.cs b/src/Whim.Tests/Store/WorkspaceSector/WorkspacePickersTests.cs
index b853cd274..c33aa7533 100644
--- a/src/Whim.Tests/Store/WorkspaceSector/WorkspacePickersTests.cs
+++ b/src/Whim.Tests/Store/WorkspaceSector/WorkspacePickersTests.cs
@@ -261,7 +261,7 @@ private static IWindow Setup_WindowPosition(IContext ctx, MutableRootSector root
 
 		workspace = workspace with
 		{
-			WindowPositions = workspace.WindowPositions.Add(
+			WindowPositions = workspace.WindowPositions.SetItem(
 				window.Handle,
 				new WindowPosition(WindowSize.Minimized, new Rectangle<int>())
 			)
diff --git a/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowEndTransform.cs b/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowEndTransform.cs
index dd867dea2..dd45d01b7 100644
--- a/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowEndTransform.cs
+++ b/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowEndTransform.cs
@@ -34,7 +34,7 @@ IWindow window
 			? workspace
 			: workspace with
 			{
-				WindowPositions = workspace.WindowPositions.Add(window.Handle, new WindowPosition()),
+				WindowPositions = workspace.WindowPositions.SetItem(window.Handle, new WindowPosition()),
 
 				// Restore in just the active layout engine. MinimizeWindowEnd is not called as part of
 				// Whim starting up.
diff --git a/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowStartTransform.cs b/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowStartTransform.cs
index 3805923e2..78ae7ff62 100644
--- a/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowStartTransform.cs
+++ b/src/Whim/Store/WorkspaceSector/Transforms/MinimizeWindowStartTransform.cs
@@ -41,7 +41,7 @@ IWindow window
 
 		workspace = workspace with
 		{
-			WindowPositions = workspace.WindowPositions.Add(window.Handle, new WindowPosition())
+			WindowPositions = workspace.WindowPositions.SetItem(window.Handle, new WindowPosition())
 		};
 
 		for (int idx = 0; idx < workspace.LayoutEngines.Count; idx++)