[R] Error: argument of length 0

[ens]

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:

> numberFires
[1] 73
> index_yr
 6955 6956 6957 6958 6959 6960 6961 6962 6963 6964 6965 6966 6967 6968 6969
6970 6971 6972 6973 6974 6975 6976 6977 .... [index: 346] 7300 7301 7302
7303 7304 7305 7306 7307 7308 7309 7310 7311 7312 7313 7314 7315 7316 7317
7318 7319 7320
> index_day
[1] 235
> index
[1] 7189
> FM100[(index-9):index,columnID]
 [1] 14.6560 14.3190 13.7730 12.0370 11.1230 10.9360 11.1180 10.3010 10.2910 
9.5018

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