diff --git a/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/BuilderInferenceSession.kt b/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/BuilderInferenceSession.kt
index 962ce7b3dcc..ba36babcfc1 100644
--- a/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/BuilderInferenceSession.kt
+++ b/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/BuilderInferenceSession.kt
@@ -206,8 +206,11 @@ class BuilderInferenceSession(
 
     override fun currentConstraintSystem() = ConstraintStorage.Empty
 
-    fun getNotFixedToInferredTypesSubstitutor(): NewTypeSubstitutor =
-        ComposedSubstitutor(commonSystem.buildCurrentSubstitutor() as NewTypeSubstitutor, createNonFixedTypeToVariableSubstitutor())
+    fun getNotFixedToInferredTypesSubstitutor(): NewTypeSubstitutor {
+        val currentSubstitutor =
+            commonSystem.buildCurrentSubstitutor().cast<NewTypeSubstitutor>().takeIf { !it.isEmpty } ?: return EmptySubstitutor
+        return ComposedSubstitutor(currentSubstitutor, createNonFixedTypeToVariableSubstitutor())
+    }
 
     override fun inferPostponedVariables(
         lambda: ResolvedLambdaAtom,