diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index 172dacf242d9..f56c2675d282 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -1500,8 +1500,9 @@ object desugar {
           // Desugar [T_1, ..., T_M] -> (P_1, ..., P_N) => R
           // Into    scala.PolyFunction { def apply[T_1, ..., T_M](x$1: P_1, ..., x$N: P_N): R }
 
-          val applyVParams = vargs.zipWithIndex.map { case (p, n) =>
-            makeSyntheticParameter(n + 1, p).withAddedFlags(mods.flags)
+          val applyVParams = vargs.zipWithIndex.map {
+            case (p: ValDef, _) => p.withAddedFlags(mods.flags)
+            case (p, n) => makeSyntheticParameter(n + 1, p).withAddedFlags(mods.flags)
           }
           RefinedTypeTree(polyFunctionTpt, List(
             DefDef(nme.apply, applyTParams, List(applyVParams), res, EmptyTree)
diff --git a/tests/neg/i6891.scala b/tests/neg/i6891.scala
new file mode 100644
index 000000000000..20361e22c044
--- /dev/null
+++ b/tests/neg/i6891.scala
@@ -0,0 +1,10 @@
+object Test {
+  trait Foo { type X }
+  class Bar extends Foo { type X = Int }
+  type F = [A <: Foo] => (f: A) => f.X => f.X
+
+  def main(args: Array[String]): Unit = {
+    val g: F = [A <: Foo] => (f: A) => (x: f.X) => x
+    println(g(new Bar)("foo")) // error
+  }
+}
diff --git a/tests/pos/i6891.scala b/tests/pos/i6891.scala
new file mode 100644
index 000000000000..8a4177a0107b
--- /dev/null
+++ b/tests/pos/i6891.scala
@@ -0,0 +1 @@
+def foo(App: [A] => (f: A) => Any): Any = ???
diff --git a/tests/run/i6891.check b/tests/run/i6891.check
new file mode 100644
index 000000000000..d00491fd7e5b
--- /dev/null
+++ b/tests/run/i6891.check
@@ -0,0 +1 @@
+1
diff --git a/tests/run/i6891.scala b/tests/run/i6891.scala
new file mode 100644
index 000000000000..5492b1337aa3
--- /dev/null
+++ b/tests/run/i6891.scala
@@ -0,0 +1,10 @@
+object Test {
+  trait Foo { type X }
+  class Bar extends Foo { type X = Int }
+  type F = [A <: Foo] => (f: A) => f.X => f.X
+
+  def main(args: Array[String]): Unit = {
+    val g: F = [A <: Foo] => (f: A) => (x: f.X) => x
+    println(g(new Bar)(1))
+  }
+}