diff --git a/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java b/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java
index febcfa9..a0e2a4a 100644
--- a/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java
+++ b/src/main/java/com/annimon/ownlang/lib/ClassInstanceValue.java
@@ -43,6 +43,15 @@ public class ClassInstanceValue implements Value {
         return thisMap.get(value);
     }
 
+    public void set(Value key, Value value) {
+        final Value v = thisMap.get(key);
+        if (v == null) {
+            throw new RuntimeException("Unable to add new field " 
+                    + key.asString() + " to class " + className);
+        }
+        thisMap.set(key, value);
+    }
+
     @Override
     public Object raw() {
         return null;
diff --git a/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java b/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java
index d013717..6ee8305 100644
--- a/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java
+++ b/src/main/java/com/annimon/ownlang/parser/ast/ContainerAccessExpression.java
@@ -73,6 +73,10 @@ public final class ContainerAccessExpression implements Expression, Accessible {
                 ((MapValue) container).set(lastIndex, value);
                 return value;
                 
+            case Types.CLASS:
+                ((ClassInstanceValue) container).set(lastIndex, value);
+                return value;
+                
             default:
                 throw new TypeException("Array or map expected. Got " + container.type());
         }