Index: lib/Support/YAMLTraits.cpp
===================================================================
--- lib/Support/YAMLTraits.cpp
+++ lib/Support/YAMLTraits.cpp
@@ -326,7 +326,12 @@
   } else if (MappingNode *Map = dyn_cast<MappingNode>(N)) {
     MapHNode *mapHNode = new MapHNode(N);
     for (KeyValueNode &KVN : *Map) {
-      ScalarNode *KeyScalar = dyn_cast<ScalarNode>(KVN.getKey());
+      Node *KeyNode = KVN.getKey();
+      ScalarNode *KeyScalar = dyn_cast<ScalarNode>(KeyNode);
+      if (!KeyScalar) {
+        setError(KeyNode, "Map key must be a scalar");
+        break;
+      }
       StringStorage.clear();
       StringRef KeyStr = KeyScalar->getValue(StringStorage);
       if (!StringStorage.empty()) {
Index: unittests/Support/YAMLIOTest.cpp
===================================================================
--- unittests/Support/YAMLIOTest.cpp
+++ unittests/Support/YAMLIOTest.cpp
@@ -84,6 +84,13 @@
   }
 }
 
+TEST(YAMLIO, TestMalformedMapRead) {
+  FooBar doc;
+  Input yin("{foo: 3; bar: 5}", nullptr, suppressErrorMessages);
+  yin >> doc;
+  EXPECT_TRUE(!!yin.error());
+}
+
 //
 // Test the reading of a yaml sequence of mappings
 //