diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
index c1a19d41d2feec..6f599811d4d58a 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
@@ -63,7 +63,7 @@ public class SkylarkSemanticsOptions extends OptionsBase implements Serializable
 
   @Option(
     name = "incompatible_disallow_set_constructor",
-    defaultValue = "false",
+    defaultValue = "true",
     category = "incompatible changes",
     documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
     effectTags = {OptionEffectTag.UNKNOWN},