diff --git a/pkg/kernel/lib/binary/ast_from_binary.dart b/pkg/kernel/lib/binary/ast_from_binary.dart
index 197d6b38551..21780933def 100644
--- a/pkg/kernel/lib/binary/ast_from_binary.dart
+++ b/pkg/kernel/lib/binary/ast_from_binary.dart
@@ -2101,6 +2101,11 @@ class BinaryBuilder {
 
   Supertype readSupertype() {
     InterfaceType type = readDartType();
+    assert(
+        type.nullability == _currentLibrary.nonNullable,
+        "In serialized form supertypes should have Nullability.legacy if they "
+        "are in a library that is opted out of the NNBD feature.  If they are "
+        "in an opted-in library, they should have Nullability.nonNullable.");
     return new Supertype.byReference(type.className, type.typeArguments);
   }
 
diff --git a/pkg/kernel/lib/binary/ast_to_binary.dart b/pkg/kernel/lib/binary/ast_to_binary.dart
index 382d3998bd6..141f1428eaa 100644
--- a/pkg/kernel/lib/binary/ast_to_binary.dart
+++ b/pkg/kernel/lib/binary/ast_to_binary.dart
@@ -54,6 +54,8 @@ class BinaryPrinter implements Visitor<void>, BinarySink {
   Set<CanonicalName> _knownCanonicalNameNonRootTops = new Set<CanonicalName>();
   Set<CanonicalName> _reindexedCanonicalNames = new Set<CanonicalName>();
 
+  Library _currentLibrary;
+
   /// Create a printer that writes to the given [sink].
   ///
   /// The BinaryPrinter will use its own buffer, so the [sink] does not need
@@ -930,6 +932,8 @@ class BinaryPrinter implements Visitor<void>, BinarySink {
 
   @override
   void visitLibrary(Library node) {
+    _currentLibrary = node;
+
     // ignore: DEPRECATED_MEMBER_USE_FROM_SAME_PACKAGE
     insideExternalLibrary = node.isExternal;
     libraryOffsets.add(getBufferOffset());
@@ -992,6 +996,8 @@ class BinaryPrinter implements Visitor<void>, BinarySink {
       writeUInt32(offset);
     }
     writeUInt32(procedureOffsets.length - 1);
+
+    _currentLibrary = null;
   }
 
   void writeLibraryDependencies(Library library) {
@@ -2068,11 +2074,11 @@ class BinaryPrinter implements Visitor<void>, BinarySink {
     // requires the nullability byte.
     if (node.typeArguments.isEmpty) {
       writeByte(Tag.SimpleInterfaceType);
-      writeByte(Nullability.nonNullable.index);
+      writeByte(_currentLibrary.nonNullable.index);
       writeNonNullReference(node.className);
     } else {
       writeByte(Tag.InterfaceType);
-      writeByte(Nullability.nonNullable.index);
+      writeByte(_currentLibrary.nonNullable.index);
       writeNonNullReference(node.className);
       writeNodeList(node.typeArguments);
     }