diff --git a/backend.native/tests/build.gradle b/backend.native/tests/build.gradle
index b19e50bd617..0c2ea8f441e 100644
--- a/backend.native/tests/build.gradle
+++ b/backend.native/tests/build.gradle
@@ -1059,6 +1059,11 @@ standaloneTest("lazy3") {
     source = "runtime/workers/lazy3.kt"
 }
 
+task mutableData1(type: KonanLocalTest) {
+    enabled = (project.testTarget != 'wasm32') // Workers need pthreads. Need exceptions
+    source = "runtime/workers/mutableData1.kt"
+}
+
 task enumIdentity(type: KonanLocalTest) {
     enabled = (project.testTarget != 'wasm32') // Workers need pthreads.
     goldValue = "true\n"
diff --git a/backend.native/tests/runtime/workers/mutableData1.kt b/backend.native/tests/runtime/workers/mutableData1.kt
new file mode 100644
index 00000000000..4cadf459db0
--- /dev/null
+++ b/backend.native/tests/runtime/workers/mutableData1.kt
@@ -0,0 +1,47 @@
+package runtime.workers.mutableData1
+
+import kotlin.native.concurrent.*
+import kotlin.test.*
+
+// See https://youtrack.jetbrains.com/issue/KT-39145
+@Test
+fun kt39145_1() = withWorker {
+    execute(TransferMode.SAFE, { "test" }) {
+        val data = MutableData(1_000)
+        val bytes = byteArrayOf(0, 10, 20, 30)
+        data.append(bytes, 0, 4)
+        assertEquals(4, data.size)
+
+        assertContentsEquals(bytes, data)
+    }.result
+}
+
+@Test
+fun kt39145_2() = withWorker {
+    val externalData = MutableData(1_000)
+    execute(TransferMode.SAFE, { externalData }) {
+        val bytes = byteArrayOf(0, 10, 20, 30)
+        it.append(bytes, 0, 4)
+        assertEquals(4, it.size)
+
+        assertContentsEquals(bytes, it)
+    }.result
+}
+
+@Test
+fun kt39145_3() {
+    val mainThreadData = MutableData(1_000)
+    val bytes = byteArrayOf(0, 10, 20, 30)
+    mainThreadData.append(bytes, 0, 4)
+    assertEquals(4, mainThreadData.size)
+
+    assertContentsEquals(bytes, mainThreadData)
+}
+
+private fun assertContentsEquals(expected: ByteArray, actual: MutableData) {
+    assertEquals(expected.size, actual.size)
+
+    expected.forEachIndexed { index, byte ->
+        assertEquals(byte, actual.get(index))
+    }
+}
\ No newline at end of file
diff --git a/runtime/src/main/kotlin/kotlin/native/concurrent/MutableData.kt b/runtime/src/main/kotlin/kotlin/native/concurrent/MutableData.kt
index 4be56b1bd8b..5f17ff70695 100644
--- a/runtime/src/main/kotlin/kotlin/native/concurrent/MutableData.kt
+++ b/runtime/src/main/kotlin/kotlin/native/concurrent/MutableData.kt
@@ -78,7 +78,7 @@ public class MutableData constructor(capacity: Int = 16) {
     public fun append(data: ByteArray, fromIndex: Int = 0, toIndex: Int = data.size): Unit = locked(lock) {
         if (fromIndex > toIndex)
             throw IndexOutOfBoundsException("$fromIndex is bigger than $toIndex")
-        if (toIndex == toIndex) return
+        if (fromIndex == toIndex) return
         val where = resizeDataLocked(this.size + (toIndex - fromIndex))
         data.copyInto(buffer, where, fromIndex, toIndex)
     }