diff --git a/core-tests/src/test/kotlin/net/corda/coretests/transactions/TransactionBuilderTest.kt b/core-tests/src/test/kotlin/net/corda/coretests/transactions/TransactionBuilderTest.kt
index af20bbba0f..083d91753f 100644
--- a/core-tests/src/test/kotlin/net/corda/coretests/transactions/TransactionBuilderTest.kt
+++ b/core-tests/src/test/kotlin/net/corda/coretests/transactions/TransactionBuilderTest.kt
@@ -158,24 +158,21 @@ class TransactionBuilderTest {
         val inputStateRef1 = StateRef(SecureHash.randomSHA256(), 0)
         val referenceState1 = TransactionState(DummyState(), DummyContract.PROGRAM_ID, notary)
         val referenceStateRef1 = StateRef(SecureHash.randomSHA256(), 1)
-        val timeWindow = TimeWindow.untilOnly(Instant.now())
         val builder = TransactionBuilder(notary)
                 .addInputState(StateAndRef(inputState1, inputStateRef1))
                 .addAttachment(SecureHash.allOnesHash)
                 .addOutputState(TransactionState(DummyState(), DummyContract.PROGRAM_ID, notary))
                 .addCommand(DummyCommandData, notary.owningKey)
                 .addReferenceState(StateAndRef(referenceState1, referenceStateRef1).referenced())
-        val inputState2 = TransactionState(DummyState(), DummyContract.PROGRAM_ID, notary)
         val inputStateRef2 = StateRef(SecureHash.randomSHA256(), 0)
-        val referenceState2 = TransactionState(DummyState(), DummyContract.PROGRAM_ID, notary)
         val referenceStateRef2 = StateRef(SecureHash.randomSHA256(), 1)
 
         // List accessors are mutable.
-        (builder.inputStates() as ArrayList).add(inputStateRef2)
-        (builder.attachments() as ArrayList).add(SecureHash.zeroHash)
-        (builder.outputStates() as ArrayList).add(TransactionState(DummyState(), DummyContract.PROGRAM_ID, notary))
-        (builder.commands() as ArrayList).add(Command(DummyCommandData, notary.owningKey))
-        (builder.referenceStates() as ArrayList).add(referenceStateRef2)
+        assertThat((builder.inputStates() as ArrayList).also { it.add(inputStateRef2) }).hasSize(2)
+        assertThat((builder.attachments() as ArrayList).also { it.add(SecureHash.zeroHash) }).hasSize(2)
+        assertThat((builder.outputStates() as ArrayList).also { it.add(TransactionState(DummyState(), DummyContract.PROGRAM_ID, notary)) }).hasSize(2)
+        assertThat((builder.commands() as ArrayList).also { it.add(Command(DummyCommandData, notary.owningKey)) }).hasSize(2)
+        assertThat((builder.referenceStates() as ArrayList).also { it.add(referenceStateRef2) }).hasSize(2)
 
         // List accessors are copies.
         assertThat(builder.inputStates()).hasSize(1)
diff --git a/core/src/main/kotlin/net/corda/core/transactions/TransactionBuilder.kt b/core/src/main/kotlin/net/corda/core/transactions/TransactionBuilder.kt
index 0be1c1acdf..6d41464edf 100644
--- a/core/src/main/kotlin/net/corda/core/transactions/TransactionBuilder.kt
+++ b/core/src/main/kotlin/net/corda/core/transactions/TransactionBuilder.kt
@@ -813,19 +813,19 @@ open class TransactionBuilder(
         this.privacySalt = privacySalt
     }
 
-    /** Returns a mutable copy of the list of input [StateRef]s. */
+    /** Returns an immutable list of input [StateRef]s. */
     fun inputStates(): List<StateRef> = ArrayList(inputs)
 
-    /** Returns a mutable copy of the list of reference input [StateRef]s. */
+    /** Returns an immutable list of reference input [StateRef]s. */
     fun referenceStates(): List<StateRef> = ArrayList(references)
 
-    /** Returns a mutable copy of the list of attachment hashes. */
+    /** Returns an immutable list of attachment hashes. */
     fun attachments(): List<AttachmentId> = ArrayList(attachments)
 
-    /** Returns a mutable copy of the list of output [TransactionState]s. */
+    /** Returns an immutable list of output [TransactionState]s. */
     fun outputStates(): List<TransactionState<*>> = ArrayList(outputs)
 
-    /** Returns a mutable copy of the list of [Command]s. */
+    /** Returns an immutable list of [Command]s. */
     fun commands(): List<Command<*>> = ArrayList(commands)
 
     /**