G21 command unknown?

I was looking at the serial.log (optionally) produced by OctoPrint and found this:

2018-12-17 07:29:29,636 - Send: N4 G21*30
2018-12-17 07:29:29,642 - Recv: echo:Unknown command: “G21”
2018-12-17 07:29:29,644 - Recv: ok N4 P15 B4

While the printer is idle, I can send a G21 and it indeed comes back as an unknown command. This seems strange to me.

TAZ 6, Marlin provided with CuraLE 3.2.32.

Any suggestions as to what might be wrong? G21 seems like a fundamental command. BTW, M503 reports:

Send: M503
Recv: echo:  G21    ; (mm)
Recv: echo:  M149 C ; Units in Celsius
Recv: echo:Filament settings: Disabled
Recv: echo:  M200 D3.00
Recv: echo:  M200 D0
Recv: echo:Steps per unit:
Recv: echo:  M92 X100.50 Y100.50 Z1600.00 E844.00
Recv: echo:Maximum feedrates (units/s):
Recv: echo:  M203 X300.00 Y300.00 Z3.00 E25.00
Recv: echo:Maximum Acceleration (units/s2):
Recv: echo:  M201 X9000 Y9000 Z100 E10000
Recv: echo:Acceleration (units/s2): P<print_accel> R<retract_accel> T<travel_accel>
Recv: echo:  M204 P500.00 R3000.00 T500.00
Recv: echo:Advanced: B<min_segment_time_us> S<min_feedrate> T<min_travel_feedrate> X<max_x_jerk> Y<max_y_jerk> Z<max_z_jerk> E<max_e_jerk>
Recv: echo:  M205 B20000 S0.00 T0.00 X8.00 Y8.00 Z0.40 E10.00
Recv: echo:Home offset:
Recv: echo:  M206 X0.00 Y0.00 Z0.00
Recv: echo:Auto Bed Leveling:
Recv: echo:  M420 S0
Recv: echo:Material heatup parameters:
Recv: echo:  M145 S0 H200 B70 F0
Recv: echo:  M145 S1 H240 B110 F0
Recv: echo:PID settings:
Recv: echo:  M301 P28.79 I1.91 D108.51
Recv: echo:  M304 P162.00 I17.00 D378.00
Recv: echo:Z-Probe Offset (mm):
Recv: echo:  M851 Z-1.27
Recv: echo:Linear Advance:
Recv: echo:  M900 K0.00
Recv: echo:Filament load/unload lengths:
Recv: echo:  M603 L40.00 U80.00
Recv: ok P15 B4

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