ggplot2 error when if statement added to a function

I have the following function which runs correctly:

plot_agg <- function(df, group_type, x_label, y_axis) {

     ggplot(df, aes_string(x = colnames(df)[1], y = colnames(df)[4], group = colnames(df)[3], fill=colnames(df)[3])) + 
                    geom_bar(stat = "identity", position = "dodge") 
 }

}

When I enter an if statement, however, as follows:

plot_agg <- function(df, group_type, x_label, y_axis) {

     if (y_axis = "wins") {
            ggplot(df, aes_string(x = colnames(df)[1], y = colnames(df)[4], group = colnames(df)[3], fill=colnames(df)[3])) + 
                    geom_bar(stat = "identity", position = "dodge") 
 }

}

it does not even load and gives me the following error:

Error: ggplot2 doesn't know how to deal with data of class function.

Any thoughts on why an if statement would cause this?