Swift3 doesn't recognise an extended version of NSString from an Obj-C static library

Question

I have an Objective-C static library.

In this library I have an extension for NSString (Additions), where I add a function called (int) indexOf: (NSString*) text, which returns an integer or -1 in case of an issue.

There is a Class in my library Foo.m like this:

#import "NSString+Additions.h"

@implementation Foo{

- (void) bar{
    NSString aString = "abcdefg";
    int anInt = [aString indexOf];
}

This all works fine when using the library in Objective-C.

If I use the compiled ObjCStaticLib.framework of this static library in a SWIFT application, the application crashes when calling int anInt = [aString indexOf]; with this exception: reason: '-[Swift._NSContiguousString indexOf:]: unrecognized selector sent to instance

What I believe is happening is that for some reason instead of using my extended NSString, it is using a native one that doesn't have my function.

I have tried importing the #import "NSString+Additions.h" in my Bridging-Header, but the error persists.

Has anyone encountered this before? Can someone suggest a workaround?