This happens over and over again. fminsearch is an OPTIMIZER. It does not do nonlinear regression. There is a difference.
An optimizer finds the minimum value of a general function of multiple parameters. So a SCALAR valued function of one or more variables.
A nonlinear regression can be turned into an optimization by forming the sum of squares of residuals, and minimizing that as an objective.
But if you just throw a vector into a tool like fminsearch, expect it to fail. fminsearch has no idea what you want to do. Computers cannot read your mind.