diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorUtils.java b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorUtils.java
index d3aa33794c5..c3295dbb0d6 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorUtils.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/DescriptorUtils.java
@@ -387,4 +387,13 @@ public class DescriptorUtils {
         Collections.sort(resultList);
         return resultList;
     }
+
+    @Nullable
+    public static ClassDescriptor getInnerClassByName(@NotNull ClassDescriptor classDescriptor, @NotNull String innerClassName) {
+        ClassifierDescriptor classifier = classDescriptor.getDefaultType().getMemberScope().getClassifier(Name.identifier(innerClassName));
+        assert classifier instanceof ClassDescriptor :
+                "Inner class " + innerClassName + " in " + classDescriptor + " should be instance of ClassDescriptor, but was: "
+                + (classifier == null ? "null" : classifier.getClass());
+        return (ClassDescriptor) classifier;
+    }
 }
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/JetStandardLibrary.java b/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/JetStandardLibrary.java
index b9c8babd32f..5c3bf472d71 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/JetStandardLibrary.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/types/lang/JetStandardLibrary.java
@@ -31,6 +31,7 @@ import org.jetbrains.jet.lang.descriptors.annotations.AnnotationDescriptor;
 import org.jetbrains.jet.lang.psi.JetFile;
 import org.jetbrains.jet.lang.resolve.AnalyzingUtils;
 import org.jetbrains.jet.lang.resolve.BindingTraceContext;
+import org.jetbrains.jet.lang.resolve.DescriptorUtils;
 import org.jetbrains.jet.lang.resolve.TopDownAnalyzer;
 import org.jetbrains.jet.lang.resolve.name.Name;
 import org.jetbrains.jet.lang.resolve.scopes.JetScope;
@@ -428,14 +429,14 @@ public class JetStandardLibrary {
 
     @NotNull
     public ClassDescriptor getMapEntry() {
-        ClassifierDescriptor entry = getMap().getDefaultType().getMemberScope().getClassifier(Name.identifier("Entry"));
+        ClassifierDescriptor entry = DescriptorUtils.getInnerClassByName(getMap(), "Entry");
         assert entry instanceof ClassDescriptor;
         return (ClassDescriptor) entry;
     }
 
     @NotNull
     public ClassDescriptor getMutableMapEntry() {
-        ClassifierDescriptor entry = getMutableMap().getDefaultType().getMemberScope().getClassifier(Name.identifier("MutableEntry"));
+        ClassifierDescriptor entry = DescriptorUtils.getInnerClassByName(getMutableMap(), "MutableEntry");
         assert entry instanceof ClassDescriptor;
         return (ClassDescriptor) entry;
     }