Stateflow Runtime Error (chart): Domain error. To compute complex results, make at least one input complex, e.g. 'power(complex(a),b)'. Script (#213)

Hello,

I am doing some simulations with Simulink using Matlab 2013b 64bit Win7. I have a bunch of Embedded Matlab Functions, which I connect through Simulink. Somewhere in the middle of the simulation I get the following error:

Stateflow Runtime Error (chart): Domain error. To compute complex results, make at least one input complex, e.g. 'power(complex(a),b)'. Script (#213)

Script (#213) points to an embedded Matlab file.

The strange thing is, that I only allow real values in "Ports and Data Manager" for all function inputs and outputs.

The line the debugger points at consist of simplified "c = a^b;", where a is positive real and b is real (debugger value).

The Error is reproducable and depends on the initial values and constants of the simulation.

I did not find any information on this error. Can someone help?