--- a/toolkit/components/glean/bindings/private/MemoryDistribution.cpp
+++ b/toolkit/components/glean/bindings/private/MemoryDistribution.cpp
@@ -46,17 +46,17 @@ GleanMemoryDistribution::TestGetValue(co
         !JS_DefineProperty(aCx, root, "values", valuesObj, JSPROP_ENUMERATE)) {
       return NS_ERROR_FAILURE;
     }
     auto& data = result.ref().values;
     for (auto iter = data.ConstIter(); !iter.Done(); iter.Next()) {
       const uint64_t bucket = iter.Key();
       const uint64_t count = iter.UserData();
       if (!JS_DefineProperty(aCx, valuesObj,
-                             nsPrintfCString("%lu", bucket).get(),
+                             nsPrintfCString("%" PRIu64, bucket).get(),
                              static_cast<double>(count), JSPROP_ENUMERATE)) {
         return NS_ERROR_FAILURE;
       }
     }
     aResult.setObject(*root);
   }
   return NS_OK;
 }