Arduino doesn't include "Serial" object

[blackketter]

Trying to build a trivial Arduino project using the sipeed-longan-nano board that prints to Serial. Getting this result:

Processing sipeed-longan-nano (board: sipeed-longan-nano; platform: gd32v-mac-unofficial; framework: arduino)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Verbose mode can be enabled via `-v, --verbose` option
CONFIGURATION: https://docs.platformio.org/page/boards/gd32v-mac-unofficial/sipeed-longan-nano.html
PLATFORM: GigaDevice GD32V Platform with Toolchain for Mac OS (unoffical fork) 1.1.0 #5304539 > Sipeed Longan Nano
HARDWARE: GD32VF103CBT6 108MHz, 32KB RAM, 128KB Flash
DEBUG: Current (altera-usb-blaster) External (altera-usb-blaster, gd-link, jlink, rv-link, sipeed-rv-debugger, um232h)
PACKAGES:
 - framework-arduino-gd32v 0.1.1
 - toolchain-gd32v-mac 9.2.0
Converting Blink.ino
LDF: Library Dependency Finder -> http://bit.ly/configure-pio-ldf
LDF Modes: Finder ~ chain, Compatibility ~ soft
Found 0 compatible libraries
Scanning dependencies...
No dependencies
Building in release mode
Compiling .pio/build/sipeed-longan-nano/src/Blink.ino.cpp.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/env_Eclipse/handlers.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/env_Eclipse/init.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/env_Eclipse/start.S.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/env_Eclipse/your_printf.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/_exit.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/close.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/fstat.c.o
/Users/dean/.platformio/packages/framework-arduino-gd32v/cores/arduino/GD32VF103_Firmware_Library/RISCV/env_Eclipse/your_printf.c: In function '__wrap_printf':
/Users/dean/.platformio/packages/framework-arduino-gd32v/cores/arduino/GD32VF103_Firmware_Library/RISCV/env_Eclipse/your_printf.c:4:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/isatty.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/lseek.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/read.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/sbrk.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/write.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/GD32VF103_Firmware_Library/RISCV/stubs/write_hex.c.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/IPAddress.cpp.o
/Users/dean/src/blink/src/Blink.ino: In function 'void setup()':
/Users/dean/src/blink/src/Blink.ino:34:3: error: 'Serial' was not declared in this scope
   Serial.begin(115200);
   ^~~~~~
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/PluggableUSB.cpp.o
/Users/dean/src/blink/src/Blink.ino:34:3: note: suggested alternative: 'Server'
   Serial.begin(115200);
   ^~~~~~
   Server
/Users/dean/src/blink/src/Blink.ino: In function 'void loop()':
/Users/dean/src/blink/src/Blink.ino:39:3: error: 'Serial' was not declared in this scope
   Serial.println("blink on");
   ^~~~~~
/Users/dean/src/blink/src/Blink.ino:39:3: note: suggested alternative: 'Server'
   Serial.println("blink on");
   ^~~~~~
   Server
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/Print.cpp.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/Stream.cpp.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/WString.cpp.o
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/main.cpp.o
*** [.pio/build/sipeed-longan-nano/src/Blink.ino.cpp.o] Error 1
Compiling .pio/build/sipeed-longan-nano/FrameworkArduino/wiring.c.o```