How to use the -I command line switch?

[tipdbmp]

// C:\libs\my_module.d
module my_module;
void foo() {}

// main.d
module main;
import my_module;

void main() {
     foo();
}

Running dmd with:
     dmd -IC:\libs main.d my_module.d

I get:
     Error: module my_module is in file 'my_module.d' which cannot 
be read
     import path[0] = C:\libs
     import path[1] = 
path\to\dmd\D\dmd2\windows\bin\..\..\src\phobos
     import path[2] = 
path\to\dmd\D\dmd2\windows\bin\..\..\src\druntime\import