Excel Writer: Unsupported Column Type '?'

Hi,

I have a workflow that flows from a [rule engine node] to a [Concatenate node] and to a [excel writer node].

I notice that after the [concatenate node] there are 2 of my columns that the type become “?” the rest are in string.

This result in the [excel writer node] throwing an error “Execute Failed” Unsupoorted column type ‘?’, Please remove corresponding colunn(s) from the input table".

Any advice on how i can solve this? I note from the execution results preview table that along the workflow nodes before the [Concatenate node] the 2 columns having issues are always showing ‘string’ typem up till the [Concatenate node] where it shows ‘?’ type.