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 924a4e1..96fe21e 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
@@ -306,7 +306,7 @@
 
   @Option(
       name = "incompatible_disallow_dict_lookup_unhashable_keys",
-      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 d798e23..1554f3a 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
@@ -284,6 +284,7 @@
           .incompatibleRestrictStringEscapes(false)
           .incompatibleDisallowDictLookupUnhashableKeys(false)
           .incompatibleUseCcConfigureFromRulesCc(false)
+          .incompatibleDisallowDictLookupUnhashableKeys(true)
           .build();
 
   /** Builder for {@link StarlarkSemantics}. All fields are mandatory. */
diff --git a/src/test/starlark/testdata/dict.sky b/src/test/starlark/testdata/dict.sky
index f1d6bbf..f2f73f0 100644
--- a/src/test/starlark/testdata/dict.sky
+++ b/src/test/starlark/testdata/dict.sky
@@ -91,17 +91,16 @@
 assert_eq("a" in dict(a=1), True)
 ---
 
-# should fail after --incompatible_disallow_dict_lookup_unhashable_keys is flipped to true
 # unhashable types
 
-{} in {} ## unhashable type: 'dict'
+{} in {} ### unhashable type: 'dict'
 ---
-[] in {} ## unhashable type: 'list'
+[] in {} ### unhashable type: 'list'
 ---
-len in {} ## unhashable type: 'function'
+len in {} ### unhashable type: 'function'
 ---
-{}.get([]) ## unhashable type: 'list'
+{}.get([]) ### unhashable type: 'list'
 ---
-dict().get({}) ## unhashable type: 'dict'
+dict().get({}) ### unhashable type: 'dict'
 ---
-{1: 2}.get(len) ## unhashable type: 'function'
+{1: 2}.get(len) ### unhashable type: 'function'