IPhone : What is nil, but not nil when inserting into NSMutableDictionary

I have run into some weird behavior on iPhone 6, iOS 8.3.

appVersion is an NSString* parameter being passed in.

  NSLog(@"A:%@:%d",appVersion,(int)appVersion.length);
  if (!appVersion)
    NSLog(@"a");
  if (appVersion == 0)
    NSLog(@"b");
  if (appVersion == nil)
    NSLog(@"c");
  if (appVersion == NULL)
    NSLog(@"d");
  if (appVersion == Nil)
    NSLog(@"e");
  if ([appVersion isEqual:[NSNull null]])
    NSLog(@"f");

  NSString* av = [NSString stringWithFormat:@"%@",appVersion];
  if ([av isEqualToString:@"(null)"])
    NSLog(@"g");
  if (((int)appVersion) == 0)
    NSLog(@"h");

  if (appVersion) {
    NSLog(@"B:%@:%d",appVersion,(int)appVersion);
    params[@"appversion"] = appVersion;
  }

The release build of the app returns:

A:(null):0
g
h
B:(null):0

and then crashes ('object cannot be nil (key: appversion)').

The debug build returns:

a
b
c
d
e
g
h

What is nil, but not nil?