--- a/js/src/jit-test/tests/basic/dumpStringRepresentation.js
+++ b/js/src/jit-test/tests/basic/dumpStringRepresentation.js
@@ -5,16 +5,19 @@ if (typeof dumpStringRepresentation !== 
   quit(0);
 
 print("Empty string:");
 dumpStringRepresentation("");
 
 print("\nResult of coercion to string:");
 dumpStringRepresentation();
 
+print("\nString with an index value:");
+dumpStringRepresentation((12345).toString());
+
 print("\ns = Simple short atom:");
 var s = "xxxxxxxx";
 dumpStringRepresentation(s);
 
 // Simple non-atom flat.
 print("\ns + s: Non-atom flat:");
 var s2 = s + s;
 dumpStringRepresentation(s2);

--- a/js/src/vm/String.cpp
+++ b/js/src/vm/String.cpp
@@ -206,17 +206,17 @@ JSString::dumpRepresentationHeader(js::G
     // copy-and-paste into a debugger.
     out.printf("((%s*) %p) length: %zu  flags: 0x%x", subclass, this, length(), flags);
     if (flags & LINEAR_BIT)             out.put(" LINEAR");
     if (flags & HAS_BASE_BIT)           out.put(" HAS_BASE");
     if (flags & INLINE_CHARS_BIT)       out.put(" INLINE_CHARS");
     if (flags & ATOM_BIT)               out.put(" ATOM");
     if (isPermanentAtom())              out.put(" PERMANENT");
     if (flags & LATIN1_CHARS_BIT)       out.put(" LATIN1");
-    if (flags & INDEX_VALUE_BIT)        out.put(" INDEX_VALUE(%u)", getIndexValue());
+    if (flags & INDEX_VALUE_BIT)        out.printf(" INDEX_VALUE(%u)", getIndexValue());
     out.putChar('\n');
 }
 
 void
 JSLinearString::dumpRepresentationChars(js::GenericPrinter& out, int indent) const
 {
     if (hasLatin1Chars()) {
         out.printf("%*schars: ((Latin1Char*) %p) ", indent, "", rawLatin1Chars());