why doesn't data.(ChannelFirst) work now? I has worked ok for years

Enter name of first channel to analyze :  F5
Enter name of second channel to analyze :  F3
Unrecognized field name "F5".

Error in SpectralRatioST1 (line 162)
veclength = length(data.(ChannelFirst));         % length of input EEG vector

>> ChannelFirst

ChannelFirst =

'F5'

>> Data.(ChannelFirst)
Unable to resolve the name 'Data.F5'.

[Don]

MATLAB Version: 9.12.0.1884302 (R2022a)

MATLAB License Number: xxxxxxxxxxxx

Operating System: Microsoft Windows 10 Pro Version 10.0 (Build 19043)

Java Version: Java 1.8.0_202-b08 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode

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MATLAB Version 9.12 (R2022a)

Signal Processing Toolbox Version 9.0 (R2022a)

Statistics and Machine Learning Toolbox Version 12.3 (R2022a)

[Jan]

What is the contents of "data"? Why is it one time in lower and the other one in upper case?