sort on multicolumn files

[P Kensche]

Hello,

I use 4.5.3 from coreutils 5.93 and have a problem with sort.

File a:
a-1     1
a       0

is sorted by "sort -k 1 a" to
a       0
a-1     1


In contrast file b:
a               a       B
a-1             b       G

is sorted by "sort -k 1 b" to
a-1             b       G
a               a       B

I assume that the sorting of file b is not correct (because it is not 
lexicographical?).
Anyway - in this combination of outputs sort unusable in combination 
with join (which is exactly what I tried to do).

Is this a bug?

Regard,
   Philip