diff --git a/src/main/scala/millfork/env/Environment.scala b/src/main/scala/millfork/env/Environment.scala
index 1cbc0586..d664f403 100644
--- a/src/main/scala/millfork/env/Environment.scala
+++ b/src/main/scala/millfork/env/Environment.scala
@@ -1004,6 +1004,8 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
               AssemblyParam(typ, RegisterVariable(reg, typ), AssemblyParameterPassingBehaviour.Copy)
             case ByZRegister(reg) =>
               AssemblyParam(typ, ZRegisterVariable(reg, typ), AssemblyParameterPassingBehaviour.Copy)
+            case ByM6809Register(reg) =>
+              AssemblyParam(typ, M6809RegisterVariable(reg, typ), AssemblyParameterPassingBehaviour.Copy)
             case ByConstant(vn) =>
               AssemblyParam(typ, Placeholder(vn, typ), AssemblyParameterPassingBehaviour.ByConstant)
             case ByReference(vn) =>
@@ -1299,6 +1301,7 @@ class Environment(val parent: Option[Environment], val prefix: String, val cpuFa
         }
       case ByMosRegister(_) => ()
       case ByZRegister(_) => ()
+      case ByM6809Register(_) => ()
       case ByConstant(name) =>
         val v = ConstantThing(prefix + name, UnexpandedConstant(prefix + name, typ.size), typ)
         addThing(v, stmt.position)
diff --git a/src/main/scala/millfork/env/Thing.scala b/src/main/scala/millfork/env/Thing.scala
index 66010646..d2903b93 100644
--- a/src/main/scala/millfork/env/Thing.scala
+++ b/src/main/scala/millfork/env/Thing.scala
@@ -225,6 +225,11 @@ case class ZRegisterVariable(register: ZRegister.Value, typ: Type) extends Varia
   override def isVolatile: Boolean = false
 }
 
+case class M6809RegisterVariable(register: M6809Register.Value, typ: Type) extends Variable {
+  def name: String = register.toString
+  override def isVolatile: Boolean = false
+}
+
 case class Placeholder(name: String, typ: Type) extends Variable {
   override def isVolatile: Boolean = false
 }
diff --git a/src/test/scala/millfork/test/BooleanSuite.scala b/src/test/scala/millfork/test/BooleanSuite.scala
index 23f14091..91a4f99e 100644
--- a/src/test/scala/millfork/test/BooleanSuite.scala
+++ b/src/test/scala/millfork/test/BooleanSuite.scala
@@ -198,4 +198,34 @@ class BooleanSuite extends FunSuite with Matchers {
       """.stripMargin
     EmuUnoptimizedRun(code).readWord(0xc000) should equal(5)
   }
+
+  test("Builtin types") {
+    val code ="""
+      | byte output @$c000
+      | noinline asm set_carry f(byte a) {
+      | #if ARCH_6502
+      |   CLC
+      |   ADC #1
+      |   RTS
+      | #elseif ARCH_I80
+      |   ADD A,1
+      |   RET
+      | #elseif ARCH_6809
+      |   ADDA #1
+      |   RTS
+      | #else
+      |   #error
+      | #endif
+      | }
+      | void main () {
+      |   output = 0
+      |   if f(0) { output += 100 }
+      |   if f($ff) { output += 1 }
+      | }
+      |
+      """.stripMargin
+    EmuUnoptimizedCrossPlatformRun(Cpu.Mos, Cpu.Z80)(code){ m =>
+      m.readByte(0xc000) should equal(1)
+    }
+  }
 }