StopCoroutine Does Nothing

I have these two coroutines:

 IEnumerator SyncTimeDifference () {
     while (TwoCharacters) {
         SecondsSinceLastSwap++;
         
         while ((SecondsSinceLastSwap > 60) && (SyncPoints != -MaxSP)) {
             StartCoroutine(SyncPenalty());
             
             yield return new WaitForSeconds(1.0F);
             
             SecondsSinceLastSwap++;
         }
         
         if (SecondsSinceLastSwap < 60) {
             StopCoroutine("SyncPenalty");
         }
         
         if (SyncPoints == -MaxSP) {
             StopCoroutine("SyncPenalty");
         }
         
         yield return new WaitForSeconds(1.0F);
     }
 }
 
 IEnumerator SyncPenalty () {
     while (SecondsSinceLastSwap > 60) {
         SyncPoints--;
         
         if (SyncPoints == -MaxSP) {
             StopCoroutine("SyncPenalty");
         }
         
         yield return new WaitForSeconds(1.0F);
     }
     
     if (SecondsSinceLastSwap <= 60 || SyncPoints == -MaxSP) {
         StopCoroutine("SyncPenalty");
         
         yield return null;
     }
 }

It's decreasing the variable at the right points, but it isn't stopping the coroutines. At all. What am I doing wrong?