diff --git a/src/ZeroSerializerGenerator.cs b/src/ZeroSerializerGenerator.cs
index 53c9e9a..af0151e 100644
--- a/src/ZeroSerializerGenerator.cs
+++ b/src/ZeroSerializerGenerator.cs
@@ -1736,6 +1736,10 @@ private static void CreateShapeTag(ITypeSymbol typeSymbol, StringBuilder shapeTa
         {
             var underlying = nullableType.TypeArguments[0];
             CreateShapeTag(underlying, shapeTagBuilder);
+            if (underlying is INamedTypeSymbol named && named.TypeKind == TypeKind.Struct && !TryGetBlittableStructByteCount(named, out _))
+            {
+                return;
+            }
             shapeTagBuilder.Append('?');
             return;
         }
diff --git a/tests/ShapeTagAndHashTests.cs b/tests/ShapeTagAndHashTests.cs
index 6a7e9bb..e2b07e5 100644
--- a/tests/ShapeTagAndHashTests.cs
+++ b/tests/ShapeTagAndHashTests.cs
@@ -48,7 +48,7 @@ public void EnumClassViewTest()
     [Fact]
     public void SchemaSignatureTestsModelViewTest()
     {
-        string schemaSignatureExpected = "v1/{blittable{int,enum:short},{int,enum:byte},blittable{int,enum:short}[],{},blittable{},{enum:byte,enum:short}?,enum:byte[],enum:ulong[],int?,bool?}";
+        string schemaSignatureExpected = "v1/{blittable{int,enum:short},{int,enum:byte},blittable{int,enum:short}[],{},blittable{},{enum:byte,enum:short},enum:byte[],enum:ulong[],int?,bool?}";
         Assert.Equal(SchemaSignatureTestsModelView.ShapeTag, schemaSignatureExpected);
         Assert.Equal(SchemaSignatureTestsModelView.ShapeHash, XXHash32.HashToUInt32(schemaSignatureExpected));
     }