How do I annotate text (containing subscript) on individual facet in ggplot2 [duplicate]

I need to annotate different facet with different labels in a faceted ggplots. Yes, I try creating an additional data frame to hold the annotations as per a response to this similar question. Annotating text on individual facet in ggplot2

The challenge is that since the annotation includes subscripts; I have had to use function 'expression' to construct them so I can then use geom_text.

ModelDisplay = data.frame(
  label = c(expression(model:0.0034+0.0011*Sph[4216]*(h)), expression(model:0.0034+0.0011*Sph[4216]*(h)), expression(model:0.0034+0.0011*Sph[4216]*(h)),expression(model:0.0034+0.0011*Sph[4216]*(h))),
  dir.hor   = c(45, 90, 135, 180)
)

This code gave the error statement:

Error in as.data.frame.default(x[[i]], optional = TRUE) : 
  cannot coerce class ""expression"" to a data.frame

Please any help would be appreciated.

Edit: Here is my ggplot2 code.

ggplot(data = allazimvariogram_EmpandMod_lagbased, aes(x = dist, y = gamma))+
  labs(x = "Lag Distance, m", y = "Porosity Empirical Variogram")+
  geom_point(color = "blue") +
  coord_cartesian(xlim = c(0, 8000), ylim = c(0, 0.008))+
  scale_x_continuous(breaks = seq(0, 8000, by = 2000))+
  scale_y_continuous(breaks = seq(0, 0.008, by = 0.001))+
  geom_smooth(aes(x=dist, y=Modgamma), se = FALSE, colour = "red")+
  geom_text(data    = ModelDisplay, mapping = aes(x = -Inf, y = -Inf, label = label),hjust   = -0.1,vjust   = -1, parse = T)+
  facet_wrap(~ dir.hor, nrow = 2)