Globals outside a UserType

I have a Function within a UserType that needs to modify a Global outside the UserType. When I try my code I get the error "Compile Error: Operator ''-'' can not be used with strings".
The variable I'm trying to modify is Energy:Float at the top.
I've been lost since BlitzWiki's been offline. What are the possible solutions?

Global Energy:Float 'Player Energy

'Projectile
Type Projectile
	Field Node:ISceneNode 'Projectile SceneNode
	Field Style:Int 'Style ID of Projectile
	Field Lifetime:Int 'Elapsed LifeTime of Projectile
	Field State:Int 'State of Projectile
	Global Count:TList = CreateList() 'List of Projectiles
	Global Info:String[][] 'Projectile Table
	
	'Use
	Function Use:Projectile( StyleID:Int )
		Local P:Projectile = New Projectile 'New Instance of Projectile
		Local ProjectileMesh:IAnimatedMesh = Scene.getMesh( "Media/" + Projectile.Info[0][StyleID] + ".ms3d" ) 'Projectile Mesh
		
		'Check Projectile Mesh
		If( ProjectileMesh >< Null And ProjectileMesh.isValid() = True )
			P.Node = Scene.addMeshSceneNode( ProjectileMesh.getMesh( 0 ), Null, Null, PlayerNode.getAbsolutePosition(), PlayerNode.GetRotation())
			
		'Projectile Mesh Invalid
		Else
			'Display Error Message
			End
		End If
		
		'Check Projectile Node
		If( P.Node.isValid() = True )
			P.Style = StyleID 'Set Style ID
			P.Lifetime = 0 'Set Initial LifeTime
			P.State = Null 'Set Initial State
			Ammo[0][Ammo[1][0]] :- 1 'Subtract Ammo
			Energy :- Projectile.Info[1][StyleID] 'Subtract Energy
			ActionTimer[Ammo[1][0]] = 1000 'Set Reload
			'Subtract Accuracy * Accuracy Radius
			AlphaIndication[0] = 100 'Apply Ammo Indicator
			
			'Insert Projectile
			ListAddLast Projectile.Count, P
			
		'Projectile Node Invalid
		Else
			'Display Error Message
			End
		End If
		
		Return P
	End Function
End Type

Oh man I just noticed I subtracting a string from a float and that was the problem.