diff --git a/compiler/src/prog8/ast/antlr/Antr2Kotlin.kt b/compiler/src/prog8/ast/antlr/Antr2Kotlin.kt
index 1bedc68e4..a0edc4879 100644
--- a/compiler/src/prog8/ast/antlr/Antr2Kotlin.kt
+++ b/compiler/src/prog8/ast/antlr/Antr2Kotlin.kt
@@ -627,7 +627,7 @@ private fun prog8Parser.When_choiceContext.toAst(): WhenChoice {
     if(stmt!=null)
         stmtBlock.add(stmt)
     val scope = AnonymousScope(stmtBlock, toPosition())
-    return WhenChoice(values, scope, toPosition())
+    return WhenChoice(values?.toMutableList(), scope, toPosition())
 }
 
 private fun prog8Parser.VardeclContext.toAst(): VarDecl {
diff --git a/compiler/src/prog8/ast/statements/AstStatements.kt b/compiler/src/prog8/ast/statements/AstStatements.kt
index 0a5f2801d..edace92bd 100644
--- a/compiler/src/prog8/ast/statements/AstStatements.kt
+++ b/compiler/src/prog8/ast/statements/AstStatements.kt
@@ -991,7 +991,7 @@ class WhenStatement(var condition: Expression,
     override fun accept(visitor: AstWalker, parent: Node) = visitor.visit(this, parent)
 }
 
-class WhenChoice(var values: List<Expression>?,           // if null,  this is the 'else' part
+class WhenChoice(var values: MutableList<Expression>?,           // if null,  this is the 'else' part
                  var statements: AnonymousScope,
                  override val position: Position) : Node {
     override lateinit var parent: Node
@@ -1008,7 +1008,9 @@ class WhenChoice(var values: List<Expression>?,           // if null,  this is t
             statements = replacement
             replacement.parent = this
         } else if(choiceValues!=null && node in choiceValues) {
-            throw FatalAstException("cannot replace choice values")
+            val idx = choiceValues.indexOf(node)
+            choiceValues[idx] = replacement as Expression
+            replacement.parent = this
         } else {
             throw FatalAstException("invalid replacement")
         }