Based on the advice of this question: no such method error: ImmutableList.copyOf()
I had to exclude the google collections jar:
<dependency>
<groupId>org.zkoss.zk</groupId>
<artifactId>zkspring-core</artifactId>
<version>3.1</version>
<exclusions>
<exclusion>
<groupId>com.google.collections</groupId>
<artifactId>google-collections</artifactId>
</exclusion>
</exclusions>
</dependency>