Shortcut ‘Ctrl-+’ not working in CodeMirror

Why shortcut ‘Ctrl-+’ not working in CodeMirror ?

import { CodemirrorComponent } from 'ng2-codemirror';
...
@ViewChild('novelEditor') private novelEditor: CodemirrorComponent;
...
const cm = this.novelEditor.instance;
const codeMirrorHotKeyMap = this.editorService.getCodeMirrorHotKeyMap(this.novelEditor);
cm.addKeyMap(codeMirrorHotKeyMap, false);
...
getCodeMirrorHotKeyMap(codeMirror: CodemirrorComponent) {
return {
'Ctrl-+': () => console.log('ctrl +'), // not working
'Ctrl--': () => console.log('ctrl -')
}
}


‘Ctrl–’, ‘Ctrl-*’, ‘Ctrl-/’,‘Ctrl-.’,‘Ctrl-0’ - working all, but only ‘Ctrl-+’ not working.

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