diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index 2cfd292fd6dd..85dd5e6665c6 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -1732,7 +1732,7 @@ object desugar {
 
           val applyVParams = vargs.zipWithIndex.map {
             case (p: ValDef, _) => p.withAddedFlags(mods.flags)
-            case (p, n) => makeSyntheticParameter(n + 1, p).withAddedFlags(mods.flags)
+            case (p, n) => makeSyntheticParameter(n + 1, p).withAddedFlags(mods.flags.toTermFlags)
           }
           RefinedTypeTree(polyFunctionTpt, List(
             DefDef(nme.apply, applyTParams :: applyVParams :: Nil, res, EmptyTree).withFlags(Synthetic)
diff --git a/tests/pos-custom-args/captures/i16871.scala b/tests/pos-custom-args/captures/i16871.scala
new file mode 100644
index 000000000000..3251a7135346
--- /dev/null
+++ b/tests/pos-custom-args/captures/i16871.scala
@@ -0,0 +1,3 @@
+import scala.language.experimental.captureChecking
+
+val f: [X] => Int => Int = [X] => (x: Int) => x
\ No newline at end of file