Layer 'fc1': Invalid input data for fully connected layer. The input data must not have both spatial and temporal dimensions.

My network:

Layers = [ ...

sequenceInputLayer([50,1,3],'Name','retina');

reluLayer('Name','relu1');

fullyConnectedLayer(400,'Name','fc1')

reluLayer('Name','relu2');

fullyConnectedLayer(200,'Name','fc2')

reluLayer('Name','relu3');

fullyConnectedLayer(100,'Name','fc3')

reluLayer('Name','relu4');

fullyConnectedLayer(64);

regressionLayer('Name','output')];

The first fully connected layers seems to think its input contains both spatial and temporal numbers:

[net,netinfo] = trainNetwork(trainInput,trainTarget,Layers,options);

Caused by:

Layer 'fc1': Invalid input data for fully connected layer. The input

data must not have both spatial and temporal dimensions.

Which is not true. It is only spatial and with three "spectral" instances per spatial point (50 in total)

How is MATLAB able to judge this?

How to fix it?