C51 FATAL-ERROR : NON-NULL ARGUMENT EXPECTED

I'm using Simplicity Studio with Keil C51 v9.53...

I tried enabling c99 compatibility by adding "--c99" to the "Additional Flags" setting within the compiler settings in project properties. Long story short: it went from complaining about my unknown array size (in a structure) to this:

C51 FATAL-ERROR - ACTION: PARSING INVOKE-/#PRAGMA-LINE LINE: C:\SiliconLabs\SimplicityStudio\v4\developer\toolchains\keil_8051\9.53\BIN\C51.exe ..\..\modbus_rtu_slave\mb_server.c OMF2 SMALL DEBUG OBJECTEXTEND ROM(LARGE) WARNINGLEVEL(2) FLOATFUZZY(3) OPTIMIZE(8,SPEED) DEFINE(DEBUG=1) INTVECTOR(0X0000) INTPROMOTE INCDIR(C:/SiliconLabs/SimplicityStudio/v4/developer/sdks/8051/v4.0.3//Device/shared/si8051base;C:\Users\clark\Projects\Controller-4\SiLabs Code\Common;C:\Users\clark\Projects\Controller-4\SiLabs Code\modbus_rtu_slave;C:\Users\clark\Projects\Controller-4\SiLabs Code\Stepper-A4988;C:/SiliconLabs/SimplicityStudio/v4/developer/sdks/8051/v4.0.3//Device/C8051F540/inc) PRINT(.\src\modbus_rtu_slave\mb_server.lst) COND PAGEWIDTH(120) PAGELENGTH(65) ERROR: NON-NULL ARGUMENT EXPECTED
C51 TERMINATED.

I believe this is the command line being issued by the builder:

C51 "..\\..\\modbus_rtu_slave\\mb_server.c" OMF2 SMALL DEBUG OBJECTEXTEND ROM(LARGE) WARNINGLEVEL (2) FLOATFUZZY(3) OPTIMIZE(8,SPEED) DEFINE (DEBUG=1) INTVECTOR(0x0000) INTPROMOTE "INCDIR(C:/SiliconLabs/SimplicityStudio/v4/developer/sdks/8051/v4.0.3//Device/shared/si8051base;C:\\Users\\clark\\Projects\\Controller-4\\SiLabs" "Code\\Common;C:\\Users\\clark\\Projects\\Controller-4\\SiLabs" "Code\\modbus_rtu_slave;C:\\Users\\clark\\Projects\\Controller-4\\SiLabs" "Code\\Stepper-A4988;C:/SiliconLabs/SimplicityStudio/v4/developer/sdks/8051/v4.0.3//Device/C8051F540/inc)" "PRINT(.\\src\\modbus_rtu_slave\\mb_server.lst)" COND PAGEWIDTH (120) PAGELENGTH (65) --c99 "OBJECT(.\\src\\modbus_rtu_slave\\mb_server.OBJ)"

What is wrong?