Why on earth is this an "uninitialized memory access"?

[Alexander_S_2]

Intel fortran compiler version 18.0.0, compiler flags -g -O0

Intel inspector 2018 build 522981

OS: OpenSuse 42.3

During memory analysis, it reports an uninitialized memory access for this part of my code:

do
    read(20,'(A)') identifier
    if(identifier .NE. 'GRID* ') exit
    corner_amount = corner_amount + 1
    backspace(unit=20)
    read(20,*) identifier, corner_number
    read(20,'(A)') ! inspector shows error for this line
    corner_number_max = max(corner_number, corner_number_max)
end do

Edit: similar problem when I activate "Analyze stack accesses"

Virtually every single one of my file opening calls throws an "uninitialized partial memory access" error. For example

open(unit=30, file='bad_triangles.dat', action='write', status='replace')