diff --git a/compilerAst/src/prog8/ast/AstToplevel.kt b/compilerAst/src/prog8/ast/AstToplevel.kt
index 8a03cfd6b..a53abb02c 100644
--- a/compilerAst/src/prog8/ast/AstToplevel.kt
+++ b/compilerAst/src/prog8/ast/AstToplevel.kt
@@ -28,7 +28,7 @@ object ParentSentinel : Node {
 
 interface IFunctionCall {
     var target: IdentifierReference
-    var args: MutableList<Expression>
+    val args: MutableList<Expression>
     val position: Position
     var parent: Node             // will be linked correctly later (late init)
 }
diff --git a/compilerAst/src/prog8/ast/expressions/AstExpressions.kt b/compilerAst/src/prog8/ast/expressions/AstExpressions.kt
index 3f20683c1..5c09eaa62 100644
--- a/compilerAst/src/prog8/ast/expressions/AstExpressions.kt
+++ b/compilerAst/src/prog8/ast/expressions/AstExpressions.kt
@@ -1047,7 +1047,7 @@ data class IdentifierReference(val nameInSource: List<String>, override val posi
 }
 
 class FunctionCallExpression(override var target: IdentifierReference,
-                             override var args: MutableList<Expression>,
+                             override val args: MutableList<Expression>,
                              override val position: Position) : Expression(), IFunctionCall {
     override lateinit var parent: Node
 
@@ -1204,7 +1204,7 @@ class ContainmentCheck(var element: Expression,
 }
 
 class BuiltinFunctionCall(override var target: IdentifierReference,
-                          override var args: MutableList<Expression>,
+                          override val args: MutableList<Expression>,
                           override val position: Position) : Expression(), IFunctionCall {
 
     val name = target.nameInSource.single()
diff --git a/compilerAst/src/prog8/ast/statements/AstStatements.kt b/compilerAst/src/prog8/ast/statements/AstStatements.kt
index 03a7f9d2a..3dc27a44a 100644
--- a/compilerAst/src/prog8/ast/statements/AstStatements.kt
+++ b/compilerAst/src/prog8/ast/statements/AstStatements.kt
@@ -663,7 +663,7 @@ class Jump(var address: UInt?,
 }
 
 class FunctionCallStatement(override var target: IdentifierReference,
-                            override var args: MutableList<Expression>,
+                            override val args: MutableList<Expression>,
                             val void: Boolean,
                             override val position: Position) : Statement(), IFunctionCall {
     override lateinit var parent: Node
@@ -1134,7 +1134,7 @@ class DirectMemoryWrite(var addressExpression: Expression, override val position
 // this is meant to eventually (?) be able to not have any FunctionCallStatement nodes to worry about anymore
 // However, if/when the codegen is moved over to use the new CodeAst (PtProgram etc. etc.) this is all moot.
 class BuiltinFunctionCallStatement(override var target: IdentifierReference,
-                                   override var args: MutableList<Expression>,
+                                   override val args: MutableList<Expression>,
                                    override val position: Position) : Statement(), IFunctionCall {
     val name: String = target.nameInSource.single()