Please see code.
Index: src/java/org/apache/poi/hssf/usermodel/HSSFCellStyle.java

    public void cloneStyleFrom(CellStyle source) {
		if(source instanceof HSSFCellStyle) {
			this.cloneStyleFrom((HSSFCellStyle)source);
		}
		throw new IllegalArgumentException("Can only clone from one HSSFCellStyle to another, not between HSSFCellStyle and XSSFCellStyle");
	}

it must be rewritten with

    public void cloneStyleFrom(CellStyle source) {
		if(source instanceof HSSFCellStyle) {
			this.cloneStyleFrom((HSSFCellStyle)source);
		} else {
		throw new IllegalArgumentException("Can only clone from one HSSFCellStyle to another, not between HSSFCellStyle and XSSFCellStyle");
                }
	}