diff --git a/idea/src/org/jetbrains/jet/plugin/project/AnalyzerFacadeWithCache.java b/idea/src/org/jetbrains/jet/plugin/project/AnalyzerFacadeWithCache.java
index 26fd05c926d..5ce07855b3a 100644
--- a/idea/src/org/jetbrains/jet/plugin/project/AnalyzerFacadeWithCache.java
+++ b/idea/src/org/jetbrains/jet/plugin/project/AnalyzerFacadeWithCache.java
@@ -50,6 +50,7 @@ import org.jetbrains.jet.plugin.util.ApplicationUtils;
 
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashSet;
 
 public final class AnalyzerFacadeWithCache {
 
@@ -131,7 +132,7 @@ public final class AnalyzerFacadeWithCache {
                             Project project = file.getProject();
 
 
-                            Collection<JetFile> files = JetFilesProvider.getInstance(project).allInScope(GlobalSearchScope.allScope(project));
+                            Collection<JetFile> files = new HashSet<JetFile>(JetFilesProvider.getInstance(project).allInScope(GlobalSearchScope.allScope(project)));
 
                             // Add requested file to the list of files for searching declarations
                             files.add(file);
diff --git a/idea/src/org/jetbrains/jet/plugin/project/PluginJetFilesProvider.java b/idea/src/org/jetbrains/jet/plugin/project/PluginJetFilesProvider.java
index f40dedc1d19..ad6ec19d663 100644
--- a/idea/src/org/jetbrains/jet/plugin/project/PluginJetFilesProvider.java
+++ b/idea/src/org/jetbrains/jet/plugin/project/PluginJetFilesProvider.java
@@ -43,9 +43,7 @@ import org.jetbrains.jet.lang.resolve.java.JetFilesProvider;
 import org.jetbrains.jet.plugin.JetFileType;
 import org.jetbrains.jet.plugin.JetPluginUtil;
 
-import java.util.Collection;
-import java.util.HashSet;
-import java.util.Set;
+import java.util.*;
 
 public class PluginJetFilesProvider extends JetFilesProvider {
     private final Project project;
@@ -145,7 +143,9 @@ public class PluginJetFilesProvider extends JetFilesProvider {
                     }
                 });
 
-        return Sets.newHashSet(Collections2.filter(jetFiles, Predicates.<JetFile>notNull()));
+        return Collections.unmodifiableCollection(
+                Sets.newHashSet(Collections2.filter(jetFiles, Predicates.<JetFile>notNull()))
+        );
     }
 
     @Override