diff --git a/fortran/src/include/primes.f90 b/fortran/src/include/primes.f90
index 2fcbac61..78282da9 100644
--- a/fortran/src/include/primes.f90
+++ b/fortran/src/include/primes.f90
@@ -87,6 +87,7 @@ subroutine prime_factor(num, factor)
             factor = num
         else
             factor = 2
+            call expand_sieve(int(sqrt(num), i18t) + 1)
             do while (factor <= num)
                 if (mod(num, factor) == 0) then
                     num = num / factor