Hello Andrey,
Please take a look on this subject:
It seems that the use of "i" as a variable name and as for imaginary value, makes some confusions, as illustrated in this post in the forum:
http://en.smath.info/forum/yaf_postsm30906_i-as-argument-in-a-function.aspx#post30906
Especially the posts #1, #4 and #15
Thanks,
Ioan