diff --git a/lib/smalloc.js b/lib/smalloc.js
index b6eb21fbc3298e..9ebbaf402d17df 100644
--- a/lib/smalloc.js
+++ b/lib/smalloc.js
@@ -5,9 +5,9 @@ const kMaxLength = smalloc.kMaxLength;
 const util = require('util');
 
 exports.alloc = alloc;
-exports.copyOnto = smalloc.copyOnto;
+exports.copyOnto = copyOnto;
 exports.dispose = dispose;
-exports.hasExternalData = smalloc.hasExternalData;
+exports.hasExternalData = hasExternalData;
 
 // don't allow kMaxLength to accidentally be overwritten. it's a lot less
 // apparent when a primitive is accidentally changed.
@@ -50,13 +50,21 @@ function alloc(n, obj, type) {
     throw new TypeError('obj must be an Object');
   }
 
+  if (Array.isArray(obj))
+    throw new TypeError('obj cannot be an array');
+  if (obj instanceof Buffer)
+    throw new TypeError('obj cannot be a Buffer');
+  if (smalloc.isTypedArray(obj))
+    throw new TypeError('obj cannot be a typed array');
+  if (smalloc.hasExternalData(obj))
+    throw new TypeError('object already has external array data');
+
   // 1 == v8::kExternalUint8Array, 9 == v8::kExternalUint8ClampedArray
   if (type < 1 || type > 9)
     throw new TypeError('unknown external array type: ' + type);
-  if (Array.isArray(obj))
-    throw new TypeError('Arrays are not supported');
   if (n > kMaxLength)
-    throw new RangeError('n > kMaxLength');
+    throw new RangeError('Attempt to allocate array larger than maximum ' +
+                         'size: 0x' + kMaxLength.toString(16) + ' elements');
 
   return smalloc.alloc(obj, n, type);
 }
@@ -70,7 +78,29 @@ function dispose(obj) {
   if (smalloc.isTypedArray(obj))
     throw new TypeError('obj cannot be a typed array');
   if (!smalloc.hasExternalData(obj))
-    throw new Error('obj has no external array data');
+    throw new TypeError('obj has no external array data');
 
   smalloc.dispose(obj);
 }
+
+
+function copyOnto(source, sourceStart, dest, destStart, copyLength) {
+  if (util.isPrimitive(source))
+    throw new TypeError('source must be an Object');
+  if (util.isPrimitive(dest))
+    throw new TypeError('dest must be an Object');
+  if (!smalloc.hasExternalData(source))
+    throw new TypeError('source has no external array data');
+  if (!smalloc.hasExternalData(dest))
+    throw new TypeError('dest has no external array data');
+
+  return smalloc.copyOnto(source, sourceStart, dest, destStart, copyLength);
+}
+
+
+function hasExternalData(obj) {
+  if (util.isPrimitive(obj))
+    return false;
+
+  return smalloc.hasExternalData(obj);
+}
diff --git a/src/smalloc.cc b/src/smalloc.cc
index 89461959dd8912..834b93a7473dd1 100644
--- a/src/smalloc.cc
+++ b/src/smalloc.cc
@@ -152,18 +152,14 @@ size_t ExternalArraySize(enum ExternalArrayType type) {
 void CopyOnto(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
 
-  if (!args[0]->IsObject())
-    return env->ThrowTypeError("source must be an object");
-  if (!args[2]->IsObject())
-    return env->ThrowTypeError("dest must be an object");
+  ASSERT(args[0]->IsObject());
+  ASSERT(args[2]->IsObject());
 
   Local<Object> source = args[0].As<Object>();
   Local<Object> dest = args[2].As<Object>();
 
-  if (!source->HasIndexedPropertiesInExternalArrayData())
-    return env->ThrowError("source has no external array data");
-  if (!dest->HasIndexedPropertiesInExternalArrayData())
-    return env->ThrowError("dest has no external array data");
+  ASSERT(source->HasIndexedPropertiesInExternalArrayData());
+  ASSERT(dest->HasIndexedPropertiesInExternalArrayData());
 
   size_t source_start = args[1]->Uint32Value();
   size_t dest_start = args[3]->Uint32Value();
@@ -266,11 +262,11 @@ void SliceOnto(const FunctionCallbackInfo<Value>& args) {
 void Alloc(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
 
+  ASSERT(args[0]->IsObject());
+
   Local<Object> obj = args[0].As<Object>();
 
-  // can't perform this check in JS
-  if (obj->HasIndexedPropertiesInExternalArrayData())
-    return env->ThrowTypeError("object already has external array data");
+  ASSERT(!obj->HasIndexedPropertiesInExternalArrayData());
 
   size_t length = args[1]->Uint32Value();
   enum ExternalArrayType array_type;
@@ -410,8 +406,8 @@ void Alloc(Environment* env,
 
 void HasExternalData(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
-  args.GetReturnValue().Set(args[0]->IsObject() &&
-                            HasExternalData(env, args[0].As<Object>()));
+  ASSERT(args[0]->IsObject());
+  args.GetReturnValue().Set(HasExternalData(env, args[0].As<Object>()));
 }