diff --git a/lib/loader/index.js b/lib/loader/index.js
index 3eb8950baa..b1e01f5323 100644
--- a/lib/loader/index.js
+++ b/lib/loader/index.js
@@ -88,7 +88,10 @@ function postInstantiate(extendedExports, instance) {
   const __pin = exports.__pin || F_NOEXPORTRUNTIME;
   const __unpin = exports.__unpin || F_NOEXPORTRUNTIME;
   const __collect = exports.__collect || F_NOEXPORTRUNTIME;
-  const __rtti_base = exports.__rtti_base || ~0; // oob if not present
+  const __rtti_base = exports.__rtti_base;
+  const getRttiCount = __rtti_base
+    ? function (arr) { return arr[__rtti_base >>> 2]; }
+    : F_NOEXPORTRUNTIME;
 
   extendedExports.__new = __new;
   extendedExports.__pin = __pin;
@@ -98,7 +101,7 @@ function postInstantiate(extendedExports, instance) {
   /** Gets the runtime type info for the given id. */
   function getInfo(id) {
     const U32 = new Uint32Array(memory.buffer);
-    const count = U32[__rtti_base >>> 2];
+    const count = getRttiCount(U32);
     if ((id >>>= 0) >= count) throw Error(`invalid id: ${id}`);
     return U32[(__rtti_base + 4 >>> 2) + id * 2];
   }
@@ -113,7 +116,7 @@ function postInstantiate(extendedExports, instance) {
   /** Gets the runtime base id for the given id. */
   function getBase(id) {
     const U32 = new Uint32Array(memory.buffer);
-    const count = U32[__rtti_base >>> 2];
+    const count = getRttiCount(U32);
     if ((id >>>= 0) >= count) throw Error(`invalid id: ${id}`);
     return U32[(__rtti_base + 4 >>> 2) + id * 2 + 1];
   }
@@ -284,7 +287,7 @@ function postInstantiate(extendedExports, instance) {
   function __instanceof(ptr, baseId) {
     const U32 = new Uint32Array(memory.buffer);
     let id = U32[ptr + ID_OFFSET >>> 2];
-    if (id <= U32[__rtti_base >>> 2]) {
+    if (id <= getRttiCount(U32)) {
       do {
         if (id == baseId) return true;
         id = getBase(id);