The behaviour of rangeOfString: you might probably not aware about

MyStringUtils.h

#import <Foundation/Foundation.h>

@interface MyStringUtils : NSObject

+ (BOOL)confuzion_stringContainsVotedKeywords:(NSString*)input;
+ (BOOL)stringContainsVotedKeywords:(NSString*)input;

@end

MyStringUtils.m

#import "MyStringUtils.h"

@implementation MyStringUtils

+ (BOOL)confuzion_stringContainsVotedKeywords:(NSString*)input
{
  NSRange votedRange = [input rangeOfString:@"voted"];
  if (votedRange.location != NSNotFound) {
      return YES;
  }
  else {
      return NO;
  }
}

+ (BOOL)stringContainsVotedKeywords:(NSString*)input
{
    if (input) {
        NSRange votedRange = [input rangeOfString:@"voted"];
        if (votedRange.location != NSNotFound) {
            return YES;
        }
    }
    return NO;
}

@end

MyStringUtilsTests.m

@interface SightingStringUtilsTests : XCTestCase

@end

@implementation SightingStringUtilsTests

- (void)setUp {
    [super setUp];
    // Put setup code here. This method is called before the invocation of each test method in the class.
}

- (void)tearDown {
    // Put teardown code here. This method is called after the invocation of each test method in the class.
    [super tearDown];
}

- (void)testStringContainsVotedKeywords
{
    NSString* testString = nil;
    XCTAssertFalse([SightingStringUtils stringContainsVotedKeywords:testString]); // test passed
    
    XCTAssertFalse([SightingStringUtils confuzion_stringContainsVotedKeywords:testString]); // test FAILED 
    // the reason it is failed, b/c [nil rangeOfString:@"some string"] returns location == 0, which is not the same as location == NSNotFound
    
    // below this line: tests passed no matter which variant of method you would choose
    testString = @"";
    XCTAssertFalse([SightingStringUtils stringContainsVotedKeywords:testString]);
    testString = @"voted";
    XCTAssertTrue([SightingStringUtils stringContainsVotedKeywords:testString]);
    testString = @"blah-blah voted blah-blah";
    XCTAssertTrue([SightingStringUtils stringContainsVotedKeywords:testString]);
}

@end