Avoiding a bad address error when implementing recursion in assembly

I am practicing recursion in assembly with the MIPS instruction set, but I always get a "bad address error"

fibonacci:
   # PROLOGUE
   subu    $sp, $sp, 8
   sw    $ra, 8($sp)
   sw    $fp, 4($sp)
   addu    $fp, $sp, 8

   # BODY
   bgt $a0, 1, rec
   move $v0, $a0
   j ret

rec:
   subu $a0, $a0, 1 #sets a0 = j-1
   subu $s0, $a0, 1 #saves s0 = j-2 for later
   jal  fibonacci         # call fibonacci
   # $v0 == f(j-1)
   move $t0, $v0 # t0 = f(j-1)
   move $a0, $s0 #sets a0 = j-2
   jal fibonacci
   add   $v0, $v0, $t0      # $v0 = f(j-2) + f(j-1) 
ret:
   # EPILOGUE
   move    $sp, $fp
   lw    $ra, ($fp)
   lw    $fp, -4($sp)
   jr     $ra

This code works fine on the inputs 1, 2, but when I try to compute f(3), I get the following:

Exception occurred at PC=0x0040004c
  Bad address in data/stack read: 0x00000004
  Exception 7  [Bad data address]  occurred and ignored

I suspect that the issue has something to do with the 2nd call to fibonacci. But what would be the issue with the 2nd call to fibonacci? Nothing looks suspicious.