diff --git a/src/MapTo/Sources/MapClassSource.cs b/src/MapTo/Sources/MapClassSource.cs
index 12074c5..73b87a1 100644
--- a/src/MapTo/Sources/MapClassSource.cs
+++ b/src/MapTo/Sources/MapClassSource.cs
@@ -33,7 +33,7 @@ namespace MapTo.Sources
                 .GeneratePrivateConstructor(model)
                 .WriteLine();
 
-            if(PropertiesAreReadOnly(model))
+            if(!PropertiesAreReadOnly(model))
             {
                 builder.GenerateUpdateMethod(model);
             }