Selectc Single-Statement doesn't work in Function Module of Web-Services

After I practiced my Webservice with an easy funtion module I wanted to do more dann only sum two parameters.

In my fm I want to select this:

SELECTmatnr FROM afpo INTO lv_matnr WHERE aufnr = iv_aufnr AND posnr = '0001'.

My fm has one importparameter: iv_aufnr type aufnr and two exportparameters: ev_aufnr TYPE aufnr and ev_matnr TYPE matnr.

That I get my two exportparameters I do this:

DATA: lv_matnr TYPE matnr.

ev_aufnr = iv_aufnr.

SELECT SINGLE matnr FROM afpo INTO lv_matnr WHERE aufnr = iv_aufnr AND posnr = '0001'.

ev_matnr = lv_matnr.

When I test my fm in my ECC everything works fine. But when I want to test it with soapUI I only get the ev_aufnr. ev_matnr is nothing because my Select Single-Statement doesn't work. After the Select the local variable is initail. In ECC it's not initial. But why?

Has somebody an idea?

Thank you and have a nice Day! 😊

Marcel