diff --git a/src/runtime/runtime-intl.cc b/src/runtime/runtime-intl.cc
index 18e635c4f7..2868ae5980 100644
--- a/src/runtime/runtime-intl.cc
+++ b/src/runtime/runtime-intl.cc
@@ -785,7 +785,10 @@ RUNTIME_FUNCTION(Runtime_StringLocaleConvertCase) {
   s = String::Flatten(s);
 
   // All the languages requiring special-handling have two-letter codes.
-  if (V8_UNLIKELY(lang_arg->length() > 2))
+  // Note that we have to check for '!= 2' here because private-use language
+  // tags (x-foo) or grandfathered irregular tags (e.g. i-enochian) would have
+  // only 'x' or 'i' when they get here.
+  if (V8_UNLIKELY(lang_arg->length() != 2))
     return ConvertCase(s, is_upper, isolate);
 
   char c1, c2;
diff --git a/test/intl/general/case-mapping.js b/test/intl/general/case-mapping.js
index feb0ff54f6..0028c4150f 100644
--- a/test/intl/general/case-mapping.js
+++ b/test/intl/general/case-mapping.js
@@ -128,6 +128,8 @@ assertEquals("abci\u0307", "aBcI\u0307".toLowerCase());
 assertEquals("abci\u0307", "aBcI\u0307".toLocaleLowerCase("fil"));
 assertEquals("abci\u0307", "aBcI\u0307".toLocaleLowerCase("zh-Hant-TW"));
 assertEquals("abci\u0307", "aBcI\u0307".toLocaleLowerCase("i-klingon"));
+assertEquals("abci\u0307", "aBcI\u0307".toLocaleLowerCase("i-enochian"));
+assertEquals("abci\u0307", "aBcI\u0307".toLocaleLowerCase("x-foobar"));
 
 // Up to 8 chars are allowed for the primary language tag in BCP 47.
 assertEquals("abci\u0307", "aBcI\u0307".toLocaleLowerCase("longlang"));