Error: argument of length 0

Problem:

 I keep getting: "Error in (index - 9):index : argument of length 0" when I
run this code in an automated loop, but when I step through it by hand, I
have no problems. It keep crashing when i<-10. I cannot figure this out to
save my life! Please help and THANK YOU in advance!

variables:

FM100 is a matrix of columns: year, day (all years set to 366 days), region
1, region 2... region N
columnID <- region # +2  
numberFires is the number of fires that occurred in the region (the code
assures there are more than 0 fires)
years is a vector of fire years as pertains to each fire counted in
numberFires
explanatory.variables$DOY is a vector of Day-of-Year for each fires start
day

code:

    #Read FM100 file
    if (REGION_SCALE ==
"psa"){FM100<-read.csv("climaggregates/fm100_psa.csv",

header=FALSE,col.names=c("year","day",psa$OBJECTID))}
    if (REGION_SCALE ==
"ecosection"){FM100<-read.csv("climaggregates/fm100_ecp.csv",

header=FALSE,col.names=c("year","day",ecosection$OBJECTID))}
    for (i in 1:numberFires){
      index_yr<-which(FM100$year==years[i])

      # file default 366 days/year w/ Feb 29 set to NAN, but DOY accounts
for leap years when 
      # assigning Day-of-year for mm-dd-yyyy
      if (years[i]%%4 != 0) {index_day<-explanatory.variables$DOY[i]+1}
      if (years[i]%%4 == 0) {index_day<-explanatory.variables$DOY[i]}
      index<-index_yr[index_day]

      # 10 day antecedent FM100 average
      explanatory.variables$FM100[i]<-mean(FM100[(index-9):index,columnID])
    }
    rm(FM100)
    print("FM100 variables created")

What's in each variable: