diff --git a/listbox.go b/listbox.go
index 66ff398..54c698b 100644
--- a/listbox.go
+++ b/listbox.go
@@ -45,20 +45,29 @@ func (l *Listbox) Append(what ...string) (err error) {
 	return nil
 }
 
-// InsertBefore inserts a new item in the Listbox before the item at the given position.
-// (TODO action on invalid index)
+// InsertBefore inserts a new item in the Listbox before the item at the given position. It panics if the given index is out of bounds.
 func (l *Listbox) InsertBefore(what string, before int) (err error) {
 	l.lock.Lock()
 	defer l.lock.Unlock()
 
+	var m []string
+
 	if l.created {
+		if before < 0 || before >= l.sysData.len() {
+			goto badrange
+		}
 		return l.sysData.insertBefore(what, before)
 	}
-	m := make([]string, 0, len(l.initItems) + 1)
+	if before < 0 || before >= len(l.initItems) {
+		goto badrange
+	}
+	m = make([]string, 0, len(l.initItems) + 1)
 	m = append(m, l.initItems[:before]...)
 	m = append(m, what)
 	l.initItems = append(m, l.initItems[before:]...)
 	return nil
+badrange:
+	panic(fmt.Errorf("index %d out of range in Listbox.InsertBefore()", before))
 }
 
 // Delete removes the given item from the Listbox.
diff --git a/test/main.go b/test/main.go
index 9ff01c1..20f5bdf 100644
--- a/test/main.go
+++ b/test/main.go
@@ -59,8 +59,13 @@ func invalidTest(c *Combobox, l *Listbox) {
 	func() {
 		defer x("Listbox.Delete > len"); c.Delete(c.Len() + 5); panic(nil)
 	}()
-	// TODO
-	_ = l
+	func() {
+		defer x("Listbox.InsertBefore < 0"); l.InsertBefore("xxx", -5); panic(nil)
+	}()
+	func() {
+		defer x("Listbox.InsertBefore > len"); l.InsertBefore("xxx", l.Len() + 5); panic(nil)
+	}()
+	// TODO add Listbox.Delete() tests when I move that over
 	MsgBox("test", "all working as intended")
 }
 
diff --git a/todo.md b/todo.md
index a4cd3a4..f03931e 100644
--- a/todo.md
+++ b/todo.md
@@ -17,13 +17,13 @@ so I don't forget:
 - Combobox/Listbox.DeleteAll
 - Combobox/Listbox.Select (with Listbox.Select allowing bulk)
 - Listbox.SelectAll
-- have Combobox.InsertBefore, Listbox.InsertBefore, Combobox.Delete, and Listbox.Delete return an error on invalid index before creation, or have them panic like an invalid array index, etc.; decide which to do as these do differnet things on different platforms by default
+- have Listbox.Delete() panic on invalid index; it does not yet due to the Mac OS X signaling issue mentioned under "super important"
 	- same for other methods that take indices, like the Stack and Grid stretchy methods
 - make the Windows implementation of message boxes run on uitask
 	- ensure MsgBoxError can run if initialization failed if things change ever
 - should Labels be selectable?
 	- should message box text be selectable on all platforms or only on those that make it the default?
-- Listbox/Combobox.Len(), Index(n)
+- Listbox/Combobox.Index(n)
 	- Index(n) is the name used by reflect.Value; use a different one?
 
 important things:
@@ -45,6 +45,7 @@ important things:
 - sometimes the size of the drop-down part of a Combobox becomes 0 or 1 or some other impossibly small value on Windows
 - make gcc (Unix)/clang (Mac OS X) pedantic about warnings/errors; also -Werror
 - make sure scrollbars in Listbox work identically on all platforms (specifically the existence and autohiding of both horizontal and vertical scrollbars)
+	- pin down this behavior; also note non-editability
 - GTK+ windows cannot be resized smaller than their controls's current sizes in their current positions; find out how to overrule that so they can be freely resized
 
 super ultra important things: