Xcode strange error when button is pressed

So my Swift program in Xcode 6.4 is working fine, until I click one of my buttons in particular. When I click it, the simulator crashes and the following is printed to the console:

2015-08-12 21:41:08.323 Unit Hero 2.0[2549:99472] -[Unit_Hero_2_0.ViewController finalUnitsButton:]: unrecognized selector sent to instance 0x7faf20675d10
2015-08-12 21:41:08.326 Unit Hero 2.0[2549:99472] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Unit_Hero_2_0.ViewController finalUnitsButton:]: unrecognized selector sent to instance 0x7faf20675d10'

*** First throw call stack:

0   CoreFoundation                      0x000000010b630c65 __exceptionPreprocess + 165
1   libobjc.A.dylib                     0x000000010d19bbb7 objc_exception_throw + 45
2   CoreFoundation                      0x000000010b6380ad -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
3   CoreFoundation                      0x000000010b58e13c ___forwarding___ + 988
4   CoreFoundation                      0x000000010b58dcd8 _CF_forwarding_prep_0 + 120
5   UIKit                               0x000000010bed0d62 -[UIApplication sendAction:to:from:forEvent:] + 75
6   UIKit                               0x000000010bfe250a -[UIControl _sendActionsForEvents:withEvent:] + 467
7   UIKit                               0x000000010bfe18d9 -[UIControl touchesEnded:withEvent:] + 522
8   UIKit                               0x000000010bf1d958 -[UIWindow _sendTouchesForEvent:] + 735
9   UIKit                               0x000000010bf1e282 -[UIWindow sendEvent:] + 682
10  UIKit                               0x000000010bee4541 -[UIApplication sendEvent:] + 246
11  UIKit                               0x000000010bef1cdc _UIApplicationHandleEventFromQueueEvent + 18265
12  UIKit                               0x000000010becc59c _UIApplicationHandleEventQueue + 2066
13  CoreFoundation                      0x000000010b564431 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
14  CoreFoundation                      0x000000010b55a2fd __CFRunLoopDoSources0 + 269
15  CoreFoundation                      0x000000010b559934 __CFRunLoopRun + 868
16  CoreFoundation                      0x000000010b559366 CFRunLoopRunSpecific + 470
17  GraphicsServices                    0x000000010f604a3e GSEventRunModal + 161
18  UIKit                               0x000000010becf8c0 UIApplicationMain + 1282
19  Unit Hero 2.0                       0x000000010b4251c7 main + 135
20  libdyld.dylib                       0x000000010d8d1145 start + 1

libc++abi.dylib: terminating with uncaught exception of type NSException (lldb)

And here is the code for the button in question:

@IBAction func newCalculationButton(sender: AnyObject)
{
    if (equation.count != 0) //empties arrays to prepare for a new calculation
    {
        equation.removeAll()
    }

    if (unitsOriginal.count != 0)
    {
        unitsOriginal.removeAll()
    }

    if (operations.count != 0)
    {
        operations.removeAll()
    }

    if (numerator.count != 0)
    {
        numerator.removeAll()
    }

    if (denominator.count != 0)
    {
        denominator.removeAll()
    }

    if (largestComponent.count != 0)
    {
        largestComponent.removeAll()
    }

    finalNumerator = nil
    finalDenominator = nil
    isNilNumerator = 0 //counter for number of nil items in numerator
    isNilDenominator = 0 //counter for number of nil items in denominator

    mainLabel.text = ""
}

What exception could it be referring to? The code is pretty simple, I'm not sure what could be wrong. Any insight is much appreciated!

Solution

Some times we copy paste button and Xcode keeps old references (actions and outlets). Check it:

Check all Outlets (IB)
Check all Actions (IB)