diff --git a/compiler/src/prog8/compiler/astprocessing/TypecastsAdder.kt b/compiler/src/prog8/compiler/astprocessing/TypecastsAdder.kt
index 97a5d1599..d9c32d969 100644
--- a/compiler/src/prog8/compiler/astprocessing/TypecastsAdder.kt
+++ b/compiler/src/prog8/compiler/astprocessing/TypecastsAdder.kt
@@ -463,14 +463,16 @@ class TypecastsAdder(val program: Program, val options: CompilationOptions, val
 
     override fun after(whenChoice: WhenChoice, parent: Node): Iterable<IAstModification> {
         val conditionDt = (whenChoice.parent as When).condition.inferType(program)
-        val values = whenChoice.values
-        values?.toTypedArray()?.withIndex()?.forEach { (index, value) ->
-            val valueDt = value.inferType(program)
-            if(valueDt!=conditionDt) {
-                val castedValue = value.typecastTo(conditionDt.getOr(DataType.UNDEFINED), valueDt.getOr(DataType.UNDEFINED), true)
-                if(castedValue.first) {
-                    castedValue.second.linkParents(whenChoice)
-                    values[index] = castedValue.second
+        if(conditionDt.isKnown) {
+            val values = whenChoice.values
+            values?.toTypedArray()?.withIndex()?.forEach { (index, value) ->
+                val valueDt = value.inferType(program)
+                if(valueDt!=conditionDt) {
+                    val castedValue = value.typecastTo(conditionDt.getOr(DataType.UNDEFINED), valueDt.getOr(DataType.UNDEFINED), true)
+                    if(castedValue.first) {
+                        castedValue.second.linkParents(whenChoice)
+                        values[index] = castedValue.second
+                    }
                 }
             }
         }
diff --git a/docs/source/todo.rst b/docs/source/todo.rst
index 8d3b14f5b..e546d5831 100644
--- a/docs/source/todo.rst
+++ b/docs/source/todo.rst
@@ -1,8 +1,6 @@
 TODO
 ====
 
-chess fails to compile (crash)
-
 remove redundant assignments in calls like this where the result registers are the same as the assigment targets:
 void, cx16.r0s, cx16.r1s = cx16.mouse_pos()
 (6502 + IR)
diff --git a/examples/test.p8 b/examples/test.p8
index aed1ac724..1f0785d0a 100644
--- a/examples/test.p8
+++ b/examples/test.p8
@@ -4,6 +4,16 @@
 
 main {
     sub start() {
-        void, cx16.r0s, cx16.r1s = cx16.mouse_pos()
+        cx16.mouse_config2(1)
+        wait_mousebutton()
+
+        sub wait_mousebutton() {
+            do {
+                cx16.r0L, void, void = cx16.mouse_pos()
+            } until cx16.r0L!=0
+            do {
+                cx16.r0L, void, void = cx16.mouse_pos()
+            } until cx16.r0L==0
+        }
     }
 }