diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt
index 2ee89bb4090..41bc5bb6cb0 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt
@@ -103,7 +103,9 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext,
     }
 
     override fun KotlinTypeMarker.typeDepth(): Int {
-        require(this is ConeKotlinType)
+        require(this is ConeKotlinType) {
+            "Incorrect type of class ${this::class.java}: $this"
+        }
         return when (this) {
             is ConeFlexibleType -> Math.max(lowerBound.typeDepthSimple(), upperBound.typeDepthSimple())
             else -> typeDepthSimple()
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/InferenceUtil.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/InferenceUtil.kt
index 4121f9bce79..da515007a17 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/InferenceUtil.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/InferenceUtil.kt
@@ -45,7 +45,11 @@ open class ConeTypeVariable(name: String) : TypeVariableMarker {
 }
 
 class InferenceComponents(val ctx: TypeSystemInferenceExtensionContextDelegate) {
-    private val approximator = object : AbstractTypeApproximator(ctx) {}
+    private val approximator = object : AbstractTypeApproximator(ctx) {
+        override fun createErrorType(message: String): SimpleTypeMarker {
+            return ConeClassErrorType(message)
+        }
+    }
     val trivialConstraintTypeInferenceOracle = TrivialConstraintTypeInferenceOracle(ctx)
     private val incorporator = ConstraintIncorporator(approximator, trivialConstraintTypeInferenceOracle)
     private val injector = ConstraintInjector(incorporator, approximator)