diff --git a/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringAvailability.kt.201 b/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringAvailability.kt.201
index 33839c73535..1e56eb2990a 100644
--- a/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringAvailability.kt.201
+++ b/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringAvailability.kt.201
@@ -16,6 +16,7 @@ import org.jetbrains.kotlin.psi.psiUtil.hasBody
 import org.jetbrains.kotlin.renderer.DescriptorRenderer
 import org.jetbrains.kotlin.types.KotlinType
 import org.jetbrains.kotlin.types.isError
+import kotlin.contracts.contract
 
 class KotlinSuggestedRefactoringAvailability(refactoringSupport: SuggestedRefactoringSupport) :
     SuggestedRefactoringAvailability(refactoringSupport)
@@ -51,6 +52,7 @@ class KotlinSuggestedRefactoringAvailability(refactoringSupport: SuggestedRefact
                 descriptorWithOldSignature.valueParameters[oldIndex].type,
                 descriptorWithNewSignature.valueParameters[newIndex].type
             )
+            // oldType and newType may not be null because arguments of refineType call were all non-null
             improvedOldParameterTypesById[id] = oldType!!
             improvedNewParameterTypesById[id] = newType!!
         }
diff --git a/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringExecution.kt.201 b/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringExecution.kt.201
index 8bd21c848fe..5729c0c7169 100644
--- a/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringExecution.kt.201
+++ b/idea/src/org/jetbrains/kotlin/idea/refactoring/suggested/KotlinSuggestedRefactoringExecution.kt.201
@@ -34,7 +34,10 @@ class KotlinSuggestedRefactoringExecution(
             KotlinTypeInfo(text = newSignature.type, isCovariant = true)
         }
 
-        require(descriptorWithNewSignature.valueParameters.size == newSignature.parameters.size)
+        require(descriptorWithNewSignature.valueParameters.size == newSignature.parameters.size) {
+            "Number of parameters of newSignature is ${newSignature.parameters.size} and of the descriptor is ${descriptorWithNewSignature.valueParameters.size}"
+        }
+
         val parameterTypeInfos = descriptorWithNewSignature.valueParameters.zip(newSignature.parameters)
             .map { (parameterDescriptor, parameterData) ->
                 val type = parameterDescriptor.varargElementType ?: parameterDescriptor.type