Reproducible sample:

<zk>
	<spreadsheet src="/blank.xlsx" hflex="1" vflex="1"
		maxrows="2000" maxcolumns="20"
		showToolbar="true" showFormulabar="true" showSheetbar="true"
		hiderowhead="true" hidecolumnhead="true" />
</zk>

Reproduce steps:
1. Click on vertical scrollbar without releasing mouse until the whole grid line disappeared, then release mouse.
2. See JS error showed.

Debug information:
This error happened when creating zss.Header object.