diff --git a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
index ebe9004..91ab07b 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
@@ -155,6 +155,7 @@
         "--incompatible_no_transitive_loads=" + rand.nextBoolean(),
         "--incompatible_remap_main_repo=" + rand.nextBoolean(),
         "--incompatible_remove_native_maven_jar=" + rand.nextBoolean(),
+        "--incompatible_string_join_requires_strings=" + rand.nextBoolean(),
         "--incompatible_use_toolchain_providers_in_java_common=" + rand.nextBoolean(),
         "--internal_skylark_flag_test_canary=" + rand.nextBoolean(),
         "--incompatible_do_not_split_linking_cmdline=" + rand.nextBoolean());
@@ -200,6 +201,7 @@
         .incompatibleNoTransitiveLoads(rand.nextBoolean())
         .incompatibleRemapMainRepo(rand.nextBoolean())
         .incompatibleRemoveNativeMavenJar(rand.nextBoolean())
+        .incompatibleStringJoinRequiresStrings(rand.nextBoolean())
         .incompatibleUseToolchainProvidersInJavaCommon(rand.nextBoolean())
         .internalSkylarkFlagTestCanary(rand.nextBoolean())
         .incompatibleDoNotSplitLinkingCmdline(rand.nextBoolean())
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java b/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
index af019ff..12d3287 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
@@ -754,4 +754,17 @@
                 + "for call to method join(elements) of 'string'",
             "','.join(elements=['foo', 'bar'])");
   }
+
+  @Test
+  public void testStringJoinRequiresStrings() throws Exception {
+    new SkylarkTest("--incompatible_string_join_requires_strings")
+        .testIfErrorContains(
+            "sequence element must be a string (got 'int')", "', '.join(['foo', 2])");
+  }
+
+  @Test
+  public void testStringJoinDoesNotRequireStrings() throws Exception {
+    new SkylarkTest("--incompatible_string_join_requires_strings=false")
+        .testEval("', '.join(['foo', 2])", "'foo, 2'");
+  }
 }