This issue occurs in the following scenario:
You start a conversation with in Microsoft Lync 2013.
You share your main monitor in the conversation.
You open an Excel worksheet on the main monitor. Then, you drag a pivot table field to an area in the worksheet. For example, to the Column Label area.
To resolve this issue, install the following update:
2881070 August 2014 update for Lync 2013 (KB2881070)