diff --git a/plugins/org.eclipse.n4js/src/org/eclipse/n4js/naming/N4JSImportedNamesAdapter.java b/plugins/org.eclipse.n4js/src/org/eclipse/n4js/naming/N4JSImportedNamesAdapter.java
index e9c128984c..d0bbcafa0a 100644
--- a/plugins/org.eclipse.n4js/src/org/eclipse/n4js/naming/N4JSImportedNamesAdapter.java
+++ b/plugins/org.eclipse.n4js/src/org/eclipse/n4js/naming/N4JSImportedNamesAdapter.java
@@ -10,10 +10,10 @@
  */
 package org.eclipse.n4js.naming;
 
-import java.util.Collections;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Set;
+import java.util.TreeSet;
 
 import org.eclipse.xtext.linking.impl.ImportedNamesAdapter;
 import org.eclipse.xtext.naming.QualifiedName;
@@ -21,6 +21,7 @@
 import org.eclipse.xtext.scoping.IScope;
 
 import com.google.common.base.Preconditions;
+import com.google.common.collect.Sets;
 
 /**
  * Adapts default implementation to not normalize qualified names to be all lower case.
@@ -72,18 +73,18 @@ public IScope wrap(IScope scope) {
 	}
 
 	@Override
-	public Set<QualifiedName> getImportedNames() {
-		return Collections.unmodifiableSet(nonNullImportedNames);
+	synchronized public TreeSet<QualifiedName> getImportedNames() {
+		return Sets.newTreeSet(nonNullImportedNames);
 	}
 
 	/** Adds given element to set of imported names. */
-	public void addImportedName(QualifiedName name) {
+	synchronized public void addImportedName(QualifiedName name) {
 		Preconditions.checkNotNull(name);
 		nonNullImportedNames.add(name);
 	}
 
 	@Override
-	public void clear() {
+	synchronized public void clear() {
 		nonNullImportedNames.clear();
 	}
 }
diff --git a/plugins/org.eclipse.n4js/src/org/eclipse/n4js/resource/N4JSResourceDescription.java b/plugins/org.eclipse.n4js/src/org/eclipse/n4js/resource/N4JSResourceDescription.java
index c2aabae30b..fc2a1102cd 100644
--- a/plugins/org.eclipse.n4js/src/org/eclipse/n4js/resource/N4JSResourceDescription.java
+++ b/plugins/org.eclipse.n4js/src/org/eclipse/n4js/resource/N4JSResourceDescription.java
@@ -22,6 +22,7 @@
 import org.apache.log4j.Logger;
 import org.eclipse.emf.ecore.EObject;
 import org.eclipse.emf.ecore.resource.Resource;
+import org.eclipse.n4js.naming.N4JSImportedNamesAdapter;
 import org.eclipse.n4js.ts.typeRefs.TypeRef;
 import org.eclipse.n4js.ts.types.ContainerType;
 import org.eclipse.n4js.ts.types.TEnum;
@@ -145,18 +146,15 @@ public Iterable<QualifiedName> getImportedNames() {
 					// also all names are collected that cannot be resolved
 
 					EcoreUtil2.resolveLazyCrossReferences(getResource(), CancelIndicator.NullImpl);
-					ImportedNamesAdapter adapter = ImportedNamesAdapter.find(getResource());
-					Iterable<QualifiedName> superImportedNames = Collections.emptySet();
-					if (adapter != null) {
-						superImportedNames = adapter.getImportedNames();
-					}
+					N4JSImportedNamesAdapter adapter = (N4JSImportedNamesAdapter) ImportedNamesAdapter
+							.find(getResource());
 
 					// use sorted set to ensure order of items
 					final SortedSet<QualifiedName> importedNames;
-					if (superImportedNames != null) {
-						importedNames = Sets.newTreeSet(superImportedNames);
-					} else {
+					if (adapter == null) {
 						importedNames = Sets.<QualifiedName> newTreeSet();
+					} else {
+						importedNames = adapter.getImportedNames();
 					}
 					// import our own module name to get a proper change notification
 					Resource resource = getResource();