"Message 'at' not understood."

Hello,
After installing Unit-Lib, I get this message at Supercollider startup (on Windows 10).

Message 'at' not understood.

Specifically, this is the whole log:

compiling class library...
	Found 833 primitives.
	Compiling directory 'C:\Program Files\SuperCollider-3.10.3\SCClassLibrary'
	Compiling directory 'C:\ProgramData\SuperCollider\Extensions'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\Extensions'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\downloaded-quarks\MathLib'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\downloaded-quarks\NetLib'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\downloaded-quarks\PopUpTreeMenu'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\downloaded-quarks\VectorSpace'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\downloaded-quarks\XML'
	Compiling directory 'C:\Users\xan_9\AppData\Local\SuperCollider\downloaded-quarks\wslib'
	numentries = 1753345 / 37341792 = 0.047
	8874 method selectors, 4208 classes
	method table size 34832104 bytes, big table size 298734336
	Number of Symbols 21517
	Byte Code Size 894109
	compiled 961 files in 1.13 seconds

Info: 5 methods are currently overwritten by extensions. To see which, execute:
MethodOverride.printAll

compile done
localhost : setting clientID to 0.
internal : setting clientID to 0.
ERROR: Message 'at' not understood.
RECEIVER:
   nil
ARGS:
   Symbol 'global'
CALL STACK:
	DoesNotUnderstandError:reportError
		arg this = <instance of DoesNotUnderstandError>
	Nil:handleError
		arg this = nil
		arg error = <instance of DoesNotUnderstandError>
	Thread:handleError
		arg this = <instance of Thread>
		arg error = <instance of DoesNotUnderstandError>
	Object:throw
		arg this = <instance of DoesNotUnderstandError>
	Object:doesNotUnderstand
		arg this = nil
		arg selector = 'at'
		arg args = [*1]
	Meta_SynthDescLib:getLib
		arg this = <instance of Meta_SynthDescLib>
		arg libname = 'global'
	SynthDef:asSynthDesc
		arg this = <instance of SynthDef>
		arg libname = 'global'
		arg keepDef = true
		var lib = nil
		var stream = <instance of CollStream>
	SynthDef:writeDefFile
		arg this = <instance of SynthDef>
		arg dir = "C:\Users\xan_9\AppData\Local..."
		arg overwrite = false
		arg mdPlugin = nil
		var desc = nil
		var defFileExistedBefore = false
	SynthDef:writeOnce
		arg this = <instance of SynthDef>
		arg dir = nil
		arg mdPlugin = nil
	Meta_UDragSource:initClass
		arg this = <instance of Meta_UDragSource>
	Meta_Class:initClassTree
		arg this = <instance of Meta_Class>
		arg aClass = <instance of Meta_UDragSource>
		var implementsInitClass = nil
	ArrayedCollection:do
		arg this = [*449]
		arg function = <instance of Function>
		var i = 239
	Meta_Class:initClassTree
		arg this = <instance of Meta_Class>
		arg aClass = <instance of Meta_Object>
		var implementsInitClass = nil
	Process:startup
		arg this = <instance of Main>
		var time = 1.1633914
	Main:startup
		arg this = <instance of Main>
		var didWarnOverwrite = false
^^ The preceding error dump is for ERROR: Message 'at' not understood.
RECEIVER: nil


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Start at the Meta_SynthDescLib:getLib and Meta_UDragSource:initClass stack entries shown in the startup log, then trace how SynthDef:asSynthDesc reaches them. Reproduce the Windows 10 startup with Unit-Lib installed; done means SuperCollider starts without the nil receiver error.