knittr: non-numeric argument to binary operator

knittr is giving me the above error. The code it is failing on is
multiplying two numeric features of a data frame. I can run the code
by hand and it works fine, but when I try to knit my document, knittr
chokes on the same line.

When kitting:

Quitting from lines 161-175 (RepData_PeerAssessment2.Rmd)
Error in storm_data$PROPDMG * storm_data$property_damage_cost_factor :
  non-numeric argument to binary operator
Calls: <Anonymous> ... handle -> withCallingHandlers -> withVisible ->
eval -> eval
Execution halted

Running the same lines manually (CTRL+Enter) from the .Rmd file:

storm_data$total_damage <-

+ (storm_data$PROPDMG * storm_data$property_damage_cost_factor) +
+ (storm_data$CROPDMG * storm_data$crop_damage_cost_factor)

str(storm_data$total_damage)

num [1:902297] 25 2.5 25 2.5 2.5 2.5 2.5 2.5 25 25 ...

Call me baffled. Any pointers are greatly appreciated at this point.

Cheers, Mark

Mark Drummond
mark at markdrummond.ca

When I get sad, I stop being sad and be Awesome instead. TRUE STORY.