rbx mongo_ext compile fail

$ gem install mongo_ext
Building native extensions.  This could take a while...
ERROR:  Error installing mongo_ext:
	ERROR: Failed to build gem native extension.

/Users/mseeger/.rvm/rubies/rbx-1.0.0-rc2/bin/rbx extconf.rb
checking for asprintf()... yes
checking for ruby/st.h... yes
checking for ruby/regex.h... yes
checking for ruby/encoding.h... no
creating Makefile

make
gcc -I. -I. -I/Users/mseeger/.rvm/rubies/rbx-1.0.0-rc2/include -I. -DHAVE_ASPRINTF -DHAVE_RUBY_ST_H -DHAVE_RUBY_REGEX_H  -ggdb3 -O2  -c buffer.c
gcc -I. -I. -I/Users/mseeger/.rvm/rubies/rbx-1.0.0-rc2/include -I. -DHAVE_ASPRINTF -DHAVE_RUBY_ST_H -DHAVE_RUBY_REGEX_H  -ggdb3 -O2  -c cbson.c
cbson.c: In function ‘write_element’:
cbson.c:278: error: invalid type argument of ‘->’
cbson.c:322: error: invalid type argument of ‘->’
cbson.c:400: error: invalid type argument of ‘->’
cbson.c:419: error: invalid type argument of ‘->’
cbson.c:419: error: invalid type argument of ‘->’
cbson.c:420: error: invalid type argument of ‘->’
cbson.c:455: error: invalid type argument of ‘->’
cbson.c: In function ‘write_doc’:
cbson.c:498: error: invalid type argument of ‘->’
cbson.c:515: error: invalid type argument of ‘->’
make: *** [cbson.o] Error 1