diff --git a/ext/mini_racer_extension/mini_racer_extension.cc b/ext/mini_racer_extension/mini_racer_extension.cc
index c38b7c9..a308f71 100644
--- a/ext/mini_racer_extension/mini_racer_extension.cc
+++ b/ext/mini_racer_extension/mini_racer_extension.cc
@@ -53,7 +53,7 @@ typedef struct {
     Local<String>* filename;
     useconds_t timeout;
     EvalResult* result;
-    long max_memory;
+    size_t max_memory;
 } EvalParams;
 
 enum IsolateFlags {
@@ -118,11 +118,11 @@ static void init_v8() {
 static void gc_callback(Isolate *isolate, GCType type, GCCallbackFlags flags) {
     if((bool)isolate->GetData(MEM_SOFTLIMIT_REACHED)) return;
 
-    long softlimit = *(long*) isolate->GetData(MEM_SOFTLIMIT_VALUE);
+    size_t softlimit = *(size_t*) isolate->GetData(MEM_SOFTLIMIT_VALUE);
 
     HeapStatistics* stats = new HeapStatistics();
     isolate->GetHeapStatistics(stats);
-    long used = stats->used_heap_size();
+    size_t used = stats->used_heap_size();
 
     if(used > softlimit) {
         isolate->SetData(MEM_SOFTLIMIT_REACHED, (void*)true);
@@ -582,7 +582,7 @@ static VALUE rb_context_eval_unsafe(VALUE self, VALUE str, VALUE filename) {
 
 	VALUE mem_softlimit = rb_iv_get(self, "@max_memory");
 	if (mem_softlimit != Qnil) {
-    	eval_params.max_memory = (long)NUM2LONG(mem_softlimit);
+        eval_params.max_memory = (size_t)NUM2ULONG(mem_softlimit);
 	}
 
 	eval_result.message = NULL;
diff --git a/lib/mini_racer.rb b/lib/mini_racer.rb
index 8be2507..94a5af5 100644
--- a/lib/mini_racer.rb
+++ b/lib/mini_racer.rb
@@ -148,7 +148,9 @@ module MiniRacer
       @max_memory = nil
       @current_exception = nil
       @timeout = options[:timeout]
-      @max_memory = options[:max_memory]
+      if options[:max_memory].is_a?(Numeric) && options[:max_memory] > 0
+        @max_memory = options[:max_memory]
+      end
       @isolate = options[:isolate] || Isolate.new(options[:snapshot])
       @disposed = false
 
diff --git a/test/mini_racer_test.rb b/test/mini_racer_test.rb
index fed7b35..53a08a0 100644
--- a/test/mini_racer_test.rb
+++ b/test/mini_racer_test.rb
@@ -283,11 +283,15 @@ raise FooError, "I like foos"
     assert_equal("Undefined Conversion", context.eval("test()"))
   end
 
-  def test_fatal_alloc
-    context = MiniRacer::Context.new(max_memory: 200000000)
-    context.attach("print", proc{|a| a})
+  def test_max_memory
+    context = MiniRacer::Context.new(max_memory: 200_000_000)
 
-    assert_raises(MiniRacer::V8OutOfMemoryError) { context.eval('var a = new Array(10000); while(true) {a = a.concat(new Array(10000)); print("loop " + a.length);}') }
+    assert_raises(MiniRacer::V8OutOfMemoryError) { context.eval('let s = 1000; var a = new Array(s); a.fill(0); while(true) {s *= 1.1; let n = new Array(Math.floor(s)); n.fill(0); a = a.concat(n); };') }
+  end
+
+  def test_negative_max_memory
+    context = MiniRacer::Context.new(max_memory: -200_000_000)
+    assert_nil(context.instance_variable_get(:@max_memory))
   end
 
   module Echo