diff --git a/samples/libcurl/src/main/kotlin/org/konan/libcurl/CUrl.kt b/samples/libcurl/src/main/kotlin/org/konan/libcurl/CUrl.kt
index efbffd566ad..a88a3e562d7 100644
--- a/samples/libcurl/src/main/kotlin/org/konan/libcurl/CUrl.kt
+++ b/samples/libcurl/src/main/kotlin/org/konan/libcurl/CUrl.kt
@@ -46,8 +46,8 @@ fun CPointer<ByteVar>.toKString(length: Int): String {
 
 fun header_callback(buffer: CPointer<ByteVar>?, size: size_t, nitems: size_t, userdata: COpaquePointer?): size_t {
     if (buffer == null) return 0
-    val header = buffer.toKString((size * nitems).toInt()).trim()
     if (userdata != null) {
+        val header = buffer.toKString((size * nitems).toInt()).trim()
         val curl = userdata.asStableRef<CUrl>().get()
         curl.header(header)
     }
@@ -57,8 +57,8 @@ fun header_callback(buffer: CPointer<ByteVar>?, size: size_t, nitems: size_t, us
 
 fun write_callback(buffer: CPointer<ByteVar>?, size: size_t, nitems: size_t, userdata: COpaquePointer?): size_t {
     if (buffer == null) return 0
-    val data = buffer.toKString((size * nitems).toInt()).trim()
     if (userdata != null) {
+        val data = buffer.toKString((size * nitems).toInt()).trim()
         val curl = userdata.asStableRef<CUrl>().get()
         curl.body(data)
     }