diff --git a/idea/src/org/jetbrains/jet/plugin/debugger/evaluate/extractFunctionForDebuggerUtil.kt b/idea/src/org/jetbrains/jet/plugin/debugger/evaluate/extractFunctionForDebuggerUtil.kt
index d10f7a99703..d8b9629d0f8 100644
--- a/idea/src/org/jetbrains/jet/plugin/debugger/evaluate/extractFunctionForDebuggerUtil.kt
+++ b/idea/src/org/jetbrains/jet/plugin/debugger/evaluate/extractFunctionForDebuggerUtil.kt
@@ -96,7 +96,7 @@ fun getFunctionForExtractedFragment(
 
         val validationResult = analysisResult.descriptor!!.validate()
         if (!validationResult.conflicts.isEmpty()) {
-            throw EvaluateExceptionUtil.createEvaluateException("Following declarations are unavailable in debug scope: ${validationResult.conflicts.keySet()?.map { it.getText() }?.makeString(",")}")
+            throw EvaluateExceptionUtil.createEvaluateException("Following declarations are unavailable in debug scope: ${validationResult.conflicts.keySet().map { it.getText() }.joinToString(",")}")
         }
 
         return validationResult.descriptor.generateFunction(true)