diff --git a/src/extensions/externalize-string-extension.cc b/src/extensions/externalize-string-extension.cc
index 9fbf329818..d98c586b8f 100644
--- a/src/extensions/externalize-string-extension.cc
+++ b/src/extensions/externalize-string-extension.cc
@@ -127,8 +127,8 @@ v8::Handle<v8::Value> ExternalizeStringExtension::IsAscii(
     return v8::ThrowException(v8::String::New(
         "isAsciiString() requires a single string argument."));
   }
-  return Utils::OpenHandle(*args[0].As<v8::String>())->IsAsciiRepresentation() ?
-      v8::True() : v8::False();
+  Handle<String> string = Utils::OpenHandle(*args[0].As<v8::String>());
+  return string->IsAsciiRepresentationUnderneath() ? v8::True() : v8::False();
 }
 
 
diff --git a/test/mjsunit/string-slices.js b/test/mjsunit/string-slices.js
index 7c40229c75..2ef10c4efc 100755
--- a/test/mjsunit/string-slices.js
+++ b/test/mjsunit/string-slices.js
@@ -190,15 +190,33 @@ assertEquals("\u03B2\u03B3\u03B4\u03B5\u03B4\u03B5\u03B6\u03B7",
     utf.substring(5,1) + utf.substring(3,7));
 
 // Externalizing strings.
-var a = "123456789" + "qwertyuiopasdfghjklzxcvbnm";
-var b = "23456789qwertyuiopasdfghjklzxcvbn"
-assertEquals(a.slice(1,-1), b);
+seq = "123456789qwertyuiopasdfghjklzxcvbnm";
+cons = "123456789" + "qwertyuiopasdfghjklzxcvbnm";
+check = "23456789qwertyuiopasdfghjklzxcvbn";
 
-assertTrue(isAsciiString(a));
-externalizeString(a, true);
-assertFalse(isAsciiString(a));
+// Externalizing a sequential string changes its encoding from ascii to
+// two-byte.  The encoding of the sliced string must reflect this change too.
+slice = seq.slice(1,-1);
+assertEquals(check, slice);
 
-assertEquals(a.slice(1,-1), b);
-assertTrue(/3456789qwe/.test(a));
-assertEquals(5, a.indexOf("678"));
-assertEquals("12345", a.split("6")[0]);
+// Seq strings can only be externalized once across multiple stress-opt runs.
+if (isAsciiString(seq)) externalizeString(seq, true);
+assertFalse(isAsciiString(seq));
+assertFalse(isAsciiString(slice));
+
+assertEquals(check, seq.slice(1,-1));
+assertEquals(check, slice);
+assertTrue(/3456789qwe/.test(seq));
+assertEquals(5, seq.indexOf("678"));
+assertEquals("12345", seq.split("6")[0]);
+
+// Externalizing a cons string changes its encoding from ascii to two-byte,
+// but the slice depending on the cons string does not, as it still points to
+// the original cons string.
+slice = cons.slice(1,-1);
+assertEquals(check, slice);
+
+assertTrue(isAsciiString(cons));
+externalizeString(cons, true);
+assertFalse(isAsciiString(cons));
+assertTrue(isAsciiString(slice));