This is how it should behave, yes.
However, it doesn't in my complicated setting.

Notice that my error message says "expects default value for keyword parameter ...".

What puzzles me in that error message is the word 'default', which doesn't appear in your error message.

After defining a matrix K, a vector b, the commands are

DSolveSys(K,b,variabel=<Q1(t),Q2(t)>);
Error, invalid input: DMat:-DSolveSys expects default value for keyword parameter begynd to be of type Vector, but received {}
undebug(DMat:-DSolveSys);
DSolveSys(K,b,variabel=<Q1(t),Q2(t)>);
 

After executing the undebug command (debug works too) the result from DSolveSys is perfect.

If I replace
begynd::Vector:={}
by
begynd::{Vector,identical({})}:={}

then there is no problem.
But why do I need to?

Why does that error message exist in Maple? There must be a reason?
I'm sorry that I cannot reproduce the error in a simple setting. 

Preben Alsholm

 

The reason I raised the question here was that 3 responses in comp.soft-sys.math.maple made me think that the problem was dependent on country specific settings. When I found out that the problem didn't exist in a Unix environment I realized that the 3 people responding might be using Unix. But you are quite right. I shall direct the question directly to Maplesoft.