title: Getting error NSInternalInconsistencyException "Argument must be non-nil"
body: <p>I run this code:</p>
  
  <pre><code>- (void)unitButtonButtonTapped:(id)sender {
      [_label setString:@"Last button: Unembossed square"];
      MilitaryUnits *target = nil;
      target = [Peasants militaryUnits];
      target.position = ccp(100, 450);
      [self addChild:target];
  }
  </code></pre>
  
  <p>And I get this error:
   <em>*</em> Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Argument must be non-nil'</p>
  
  <p>These are my .h and .m class files</p>
  
  <pre><code>#import "cocos2d.h"
  
  @interface MilitaryUnits : CCSprite {
      int _experience;
      int _number_of_units;
      int _stamina;
      int _armor_level;
      int _weapon_levell;
  }
  
  @property (nonatomic, assign) int experience;
  @property (nonatomic, assign) int number_of_units;
  @property (nonatomic, assign) int stamina;
  @property (nonatomic, assign) int armor_level;
  @property (nonatomic, assign) int weapon_levell;
  
  @end
  
  @interface Peasants : MilitaryUnits{
  
  }
  +(id)militaryUnits;
  
  @end
  
  #import "MilitaryUnits.h"
  
  @implementation MilitaryUnits
  
  @synthesize number_of_units = _number_of_units;
  @synthesize stamina = _stamina;
  @synthesize experience = _experience;
  @synthesize armor_level = _armor_level;
  @synthesize weapon_levell = _weapon_levell;
  
  @end
  
  @implementation Peasants
  
  + (id)militaryUnits {
  
      Peasants *militaryUnits = nil;
      if ((militaryUnits = [[[super alloc] initWithFile:@"Target.png"] autorelease])) {
      }
      return militaryUnits;
  
  }
  
  @end
  </code></pre>
  
  <p>Note, I'm using cocos 2d</p>