diff --git a/site/docs/skylark/backward-compatibility.md b/site/docs/skylark/backward-compatibility.md
index 3857d7c..291950f 100644
--- a/site/docs/skylark/backward-compatibility.md
+++ b/site/docs/skylark/backward-compatibility.md
@@ -71,7 +71,7 @@
 with Python. A possible workaround is to use the `.update` method instead.
 
 *   Flag: `--incompatible_disallow_dict_plus`
-*   Default: `false`
+*   Default: `true`
 *   Tracking issue: [#6461](https://github.com/bazelbuild/bazel/issues/6461)
 
 ### Load must appear at top of file
diff --git a/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java
index 231480a..78b70e6 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/StarlarkSemanticsOptions.java
@@ -258,7 +258,7 @@
 
   @Option(
       name = "incompatible_disallow_dict_plus",
-      defaultValue = "false",
+      defaultValue = "true",
       documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
       effectTags = {OptionEffectTag.BUILD_FILE_SEMANTICS},
       metadataTags = {
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
index f075118..fd40dd5 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
@@ -218,7 +218,7 @@
           .incompatibleDisableDeprecatedAttrParams(false)
           .incompatibleDisableObjcProviderResources(false)
           .incompatibleDisallowDataTransition(true)
-          .incompatibleDisallowDictPlus(false)
+          .incompatibleDisallowDictPlus(true)
           .incompatibleDisallowFileType(true)
           .incompatibleDisallowLegacyJavaProvider(false)
           .incompatibleDisallowLegacyJavaInfo(false)