Steps to reproduce:
1. Download and install "Sogou Pinyin"
2. In any Unity project create canvas and add input field to it
3. Play the project and select the input field
4. Type in 'a' letter and don't press enter. Click anywhere else to lose focus of the field
5. Click on the field again and notice that more letters have appeared in the input field with the error "ArgumentOutOfRangeException: Cannot be negative"

Expected result - No extra letters, no errors shown.

Notes:
The quick download link for "Sogou Pinyin" is "http://cdn2.ime.sogou.com/6fa2aeec64fb0a6e2f7611bb25a11fe4/56275883/dl/index/1445400480/sogou_pinyin_78a.exe". This is from its official website, safe to download.

Tested and reproduced with:
Windows 7 and 5.2.2p3