Most probably because you did a "positive feedback".
Try do this
FEEDBACK Feedback connection of two input/output systems. M = FEEDBACK(M1,M2) computes a closed-loop model M for the feedback loop: u --->O---->[ M1 ]----+---> y | | y = M * u +-----[ M2 ]<---+ Negative feedback is assumed and the model M maps u to y. To apply positive feedback, use the syntax M = FEEDBACK(M1,M2,+1). M = FEEDBACK(M1,M2,FEEDIN,FEEDOUT,SIGN) builds the more general feedback interconnection: +------+ v --------->| |--------> z | M1 | u --->O---->| |----+---> y | +------+ | | | +-----[ M2 ]<---+ The vector FEEDIN contains indices into the input vector of M1 and specifies which inputs u are involved in the feedback loop. Similarly, FEEDOUT specifies which outputs y of M1 are used for feedback. If SIGN=1 then positive feedback is used. If SIGN=-1 or SIGN is omitted, then negative feedback is used. In all cases, the resulting model M has the same inputs and outputs as M1 (with their order preserved). If M1 and M2 are arrays of models, FEEDBACK returns a model array M of the same dimensions where M(:,:,k) = FEEDBACK(M1(:,:,k),M2(:,:,k)) . For dynamic systems SYS1 and SYS2, SYS = FEEDBACK(SYS1,SYS2,'name') connects SYS1 and SYS2 by matching their I/O names. The I/O names of SYS1 and SYS2 must be fully defined. See also LFT, PARALLEL, SERIES, CONNECT, INPUTOUTPUTMODEL, DYNAMICSYSTEM. Documentation for feedback doc feedback Other uses of feedback iddata/feedback InputOutputModel/feedback lti/feedback idmodel/feedback