diff --git a/src/servers/app/Window.cpp b/src/servers/app/Window.cpp
index 8928f0a72e..c986b3bbf8 100644
--- a/src/servers/app/Window.cpp
+++ b/src/servers/app/Window.cpp
@@ -1436,6 +1436,9 @@ Window::HasModal() const
 /*!	\brief Returns the windows that's in behind of the backmost position
 		this window can get.
 	Returns NULL is this window can be the backmost window.
+
+	\param workspace the workspace on which this check should be made. If
+		the value is -1, the window's current workspace will be used.
 */
 Window*
 Window::Backmost(Window* window, int32 workspace)
@@ -1443,6 +1446,10 @@ Window::Backmost(Window* window, int32 workspace)
 	if (workspace == -1)
 		workspace = fCurrentWorkspace;
 
+	ASSERT(workspace != -1);
+	if (workspace == -1)
+		return NULL;
+
 	// Desktop windows are always backmost
 	if (fFeel == kDesktopWindowFeel)
 		return NULL;
@@ -1465,6 +1472,9 @@ Window::Backmost(Window* window, int32 workspace)
 /*!	\brief Returns the windows that's in front of the frontmost position
 		this window can get.
 	Returns NULL if this window can be the frontmost window.
+
+	\param workspace the workspace on which this check should be made. If
+		the value is -1, the window's current workspace will be used.
 */
 Window*
 Window::Frontmost(Window* first, int32 workspace)
@@ -1472,6 +1482,10 @@ Window::Frontmost(Window* first, int32 workspace)
 	if (workspace == -1)
 		workspace = fCurrentWorkspace;
 
+	ASSERT(workspace != -1);
+	if (workspace == -1)
+		return NULL;
+
 	if (fFeel == kDesktopWindowFeel)
 		return first ? first : NextWindow(workspace);