ggplot question

Hello List,
I use ggplot to draw a stack bar chart. but I error message. please look it below:

ggplot(s8_plot, aes(fill=GTresult, y=cases, x=gc_label) + 

+   geom_bar(position="stack", stat="identity"))

Error: Mapping should be created with `aes()` or `aes_()`.

GTresult and gc_label are character variables, cases is numeric variable. How to fix the problem?
Thank you
Kai