ipdl compile error

I'm trying to compile my ipdl file to c++ files(make -C
$OBJDIR/ipc/ipdl), but I get the following error:

/home/zemei/mozilla/src/modules/libpref/ipc/Preferences.ipdl:14:
error: argument typename `String' of message `GetCharPref' has not
been declared
Specification is not well typed.

First I tried with non-capital s, but that has the same problem.
After that i check the ipc/test-harness/Test.ipdl file.
There are String types with capital S, but don't know what's the problem.