Why is Spring taking a perfectly good Set value and passing along a null?

Why is Spring taking a perfectly good Set value and passing along a null to the property's setter? That is, I had a simple bean declared like the following
<bean name="set" class="java.util.HashSet">
...
</bean>

<bean name="x" class="...">
<property name="y">
<map>
<entry key="z" value-ref="set"/>
...
</map>
</property>
</bean>
Where the property's setter is declared
void setY( Map<String,Object> y )
But every time setY() was called the key z's value was null. The problem's answer was very hard to track down. It comes from how Spring automatically converts objects to type specific property values.

Spring has two ways of converting a value for use in a property's setter method. The first is based on converting a textual representation to an object via the java.beans.PropertyEditorSupport API. The second is based on Spring's own org.springframework.core.convert.converter.Converter API. I has used a Converter in an unrelated part of my application's Spring context. This was my application's first use of the Converter API. Turning the Converter API on, however, brought with it other conversions I was not expecting. The most detrimental was the Collection to Object converter.

When Spring sees that the property expects an Object it tries to convert any value to an Object. (Even though every value is an object.) The Collection to Object converter converts an empty collection to null. Let me say that again .. with emphasis. The Collection to Object converter converts an empty collection to null. My set was empty. And so, a perfectly good empty set value was converted to a null. The correction is the change the setter for property y to
void setY( Map<String,?> y )
And all works now.

p.s. I would never have been able to debug this without Spring having open source. Thanks VWware, Spring Source, and all the contributors.