undefined control sequence in equation

I have loaded the amsmath package. I am not able to get the mass function of a negative binomial distribution. The code is:

\begin{equation*}

f(count_{i,t}|\lambda_{i,t},\theta_i) =
{\Gamma(\lambda_{i,t} + count_{i,t})/{\Gamma(\lambda_{i,t})\Gamma(count_{i,t}+1)}}{(\theta_i/{1+\theta_i})}^count_{i,t} {(1/{1+\theta_i})}^\lamba_{i,t}

\end{equation*}


The error is:

[14] [15] [16] [17]
! Undefined control sequence.
l.380 ...})}^count_{i,t} {(1/{1+\theta_i})}^\lamba
_{i,t}


-
Welcome to TeX.SX! You can have a look at our starter guide to familiarize yourself further with our format. –  karlkoeller Nov 23 '13 at 8:48
You have \lamba instead of \lambda. –  Przemysław Scherwentke Nov 23 '13 at 8:48
And you cannot have empty lines in an equation. –  Torbjørn T. Nov 23 '13 at 8:53
You may also be missing curly braces, { and }, around the final count_{i,t} expression, which looks like it should be placed in a superscript (exponent) position. Incidentally, even after you've made the changes made in these comments, the equation is not going to look anything like the image you've posted since you're using inline-fraction expressions rather than \frac. –  Mico Nov 23 '13 at 9:25

\documentclass{article}
\usepackage{amsmath}

\begin{document}

I would also change | to \mid : doing so will create better spacing around the symbol. –  Mico Nov 23 '13 at 16:16