Step function

I am having problem using the step function for a linear regression model.  I've created an initial model containing only the intercept.  Then using the step function, I've selected three variables to be considered for the model.

x0.lm<- lm(MEDV~1, data = x)

anova(x0.lm)

Analysis of Variance Table

Response: MEDV
           Df Sum Sq Mean Sq F value Pr(>F)
Residuals 505  42716      85

step(x0.lm, ~ CRIM + ZN + RM)

Start:  AIC= 2246.51 
 MEDV ~ 1 

Error in eval(expr, envir, enclos) : Object "MEDV" not found

I am not clear as to why the MEDV object ( the variable I am trying to predict) is not found.  I receive an identical message using the command > step(x0.lm).  

I am using R version 1.1.1.  I can run all the examples successfully.  Also, I realize that there are differences in the algorithms, but I can run the code above  on Splus version 5.  

Thanks 

Matt Pocernich 
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