Here is part of my code which fails : 

```
<aura:iteration items="{!v.layout}" var="item">
        <aura:if istrue="{!(item.elementType == 'HEADER') ? true : false}">

```

I tried rewriting this like this also :

```
 <aura:iteration items="{!v.layout}" var="item">
        <aura:if istrue="{!item.elementType == 'HEADER'}">

```

But i'm obtaining the same error. Seems like it don't like the item.elementType, where elementType is an attribute of the item instance.