This message is generated when the indicated expression is expected to be a matrix.
For the purpose of this message an expression is a matrix if the expression is a list with at least one element, all of the elements are lists of the same length, and none of the elements of the inner lists are lists.
This message can be generated by most linear algebra functions and by graphics functions, such as PseudoInverse, that use matrix arguments.
Off[message] switches off the message; On[message] switches it on. For example: Off[General::matrix].
Examples
Basic Examples(1)
The argument of PseudoInverse is expected to be a matrix: