Jenkins pipeline as code is not accepting Maps returning from Groovy scripts

Error description :

an exception which occurred:
in field groovy.lang.GString.values
in object org.codehaus.groovy.runtime.GStringImpl@1d813e5d
in field groovy.lang.Closure.delegate
in object org.jenkinsci.plugins.workflow.cps.CpsClosure2@1f68bd3
in field org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.closures
in object org.jenkinsci.plugins.workflow.cps.CpsThreadGroup@1a86f95
in object org.jenkinsci.plugins.workflow.cps.CpsThreadGroup@1a86f95
Caused: java.io.NotSerializableException: java.util.HashMap$Node
at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:860)
at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:778)
at org.jboss.marshalling.river.RiverMarshaller.doWriteFields(RiverMarshaller.java:1032)
at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:988)
at org.jboss.marshalling.river.RiverMarshaller.doWriteSerializableObject(RiverMarshaller.java:967)
at org.jboss.marshalling.river.RiverMarshaller.doWriteObject(RiverMarshaller.java:854)
at org.jboss.marshalling.river.BlockMarshaller.doWriteObject(BlockMarshaller.java:65)
at org.jboss.marshalling.river.BlockMarshaller.writeObject(BlockMarshaller.java:56)
at org.jboss.marshalling.MarshallerObjectOutputStream.writeObjectOverride(MarshallerObjectOutputStream.java:50)
at org.jboss.marshalling.river.RiverObjectOutputStream.writeObjectOverride(RiverObjectOutputStream.java:179)
at java.io.ObjectOutputStream.writeObject(Unknown Source)
at java.util.LinkedHashMap.internalWriteEntries(Unknown Source)
at java.util.HashMap.writeObject(Unknown Source)
at sun.reflect.GeneratedMethodAccessor155.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)