mod_flite fails due to undefined symbol: cmu_lex_init

Hi all, I build and run FreeSwitch 1.6.20 from source on Ubuntu 16.0.4 xenial, the mod_filte failed while loading, 2018-08-30 19:03:30.786468 [CRIT] switch_loadable_module.c:1522 Error Loading module /home/kxie/freeswitch-dev-install/lib/freeswitch/mod/mod_flite.so**/usr/local/lib/libflite_cmu_us_slt.so.1: undefined symbol: cmu_lex_init** Any idea to solve this issue? Many thanks, /Kaiduan