diff --git a/src/main/scala/millfork/env/Constant.scala b/src/main/scala/millfork/env/Constant.scala
index 51b05e6d..22b4006c 100644
--- a/src/main/scala/millfork/env/Constant.scala
+++ b/src/main/scala/millfork/env/Constant.scala
@@ -66,7 +66,7 @@ sealed trait Constant {
   }
 
   def subbyte(index: Int): Constant = {
-    if (requiredSize <= index) Constant.Zero
+    if (requiredSize > 0 && requiredSize <= index) Constant.Zero
     else index match {
       case 0 => if (isProvablyDivisibleBy256) Constant.Zero else loByte
       case 1 => hiByte
diff --git a/src/test/scala/millfork/test/StructSuite.scala b/src/test/scala/millfork/test/StructSuite.scala
index cf591e55..f9dce740 100644
--- a/src/test/scala/millfork/test/StructSuite.scala
+++ b/src/test/scala/millfork/test/StructSuite.scala
@@ -176,4 +176,30 @@ class StructSuite extends FunSuite with Matchers {
       m.readWord(0xc002) should equal(code.count(_ == '↑') + 8)
     }
   }
+
+
+  test("Boolean fields") {
+    val code =
+      """
+        |struct Sprite {
+        |    byte size,
+        |    byte size2,
+        |    byte size3,
+        |    byte size4,
+        |    bool x1,
+        |    bool x,
+        |    bool y,
+        |    bool z
+        |}
+        |
+        |array(Sprite) sprites [20] @ $c000
+        |
+        |void main() {
+        |    sprites[0].x = true
+        |}
+        |""".stripMargin
+    EmuUnoptimizedCrossPlatformRun(Cpu.Mos, Cpu.Z80, Cpu.Intel8086, Cpu.Motorola6809)(code){ m =>
+      m.readByte(0xc005) should equal(1)
+    }
+  }
 }