diff --git a/v8js_class.cc b/v8js_class.cc
index 17bdeaf..43f8bfe 100644
--- a/v8js_class.cc
+++ b/v8js_class.cc
@@ -654,7 +654,7 @@ static void v8js_compile_script(zval *this_ptr, const zend_string *str, const ze
 	return;
 }
 
-static void v8js_execute_script(zval *this_ptr, v8js_script *res, long flags, long time_limit, long memory_limit, zval **return_value)
+static void v8js_execute_script(zval *this_ptr, v8js_script *res, long flags, long time_limit, size_t memory_limit, zval **return_value)
 {
 	v8js_ctx *c = Z_V8JS_CTX_OBJ_P(this_ptr);
 
@@ -702,13 +702,19 @@ static PHP_METHOD(V8Js, executeString)
 		return;
 	}
 
+	if (memory_limit < 0) {
+		zend_throw_exception(php_ce_v8js_exception,
+				"memory_limit must not be negative", 0);
+		return;
+	}
+
 	v8js_compile_script(getThis(), str, identifier, &res);
 	if (!res) {
 		RETURN_FALSE;
 	}
 
 	zend_try {
-		v8js_execute_script(getThis(), res, flags, time_limit, memory_limit, &return_value);
+		v8js_execute_script(getThis(), res, flags, time_limit, static_cast<size_t>(memory_limit), &return_value);
 		v8js_script_free(res);
 	}
 	zend_catch {
@@ -757,11 +763,17 @@ static PHP_METHOD(V8Js, executeScript)
 		return;
 	}
 
+	if (memory_limit < 0) {
+		zend_throw_exception(php_ce_v8js_exception,
+				"memory_limit must not be negative", 0);
+		return;
+	}
+
 	if((res = (v8js_script *)zend_fetch_resource(Z_RES_P(zscript), PHP_V8JS_SCRIPT_RES_NAME, le_v8js_script)) == NULL) {
 		RETURN_FALSE;
 	}
 
-	v8js_execute_script(getThis(), res, flags, time_limit, memory_limit, &return_value);
+	v8js_execute_script(getThis(), res, flags, time_limit, static_cast<size_t>(memory_limit), &return_value);
 }
 /* }}} */
 
@@ -907,14 +919,20 @@ static PHP_METHOD(V8Js, setMemoryLimit)
 		return;
 	}
 
+	if (memory_limit < 0) {
+		zend_throw_exception(php_ce_v8js_exception,
+				"memory_limit must not be negative", 0);
+		return;
+	}
+
 	c = Z_V8JS_CTX_OBJ_P(getThis());
-	c->memory_limit = memory_limit;
+	c->memory_limit = static_cast<size_t>(memory_limit);
 
 	V8JSG(timer_mutex).lock();
 	for (std::deque< v8js_timer_ctx* >::iterator it = V8JSG(timer_stack).begin();
 		 it != V8JSG(timer_stack).end(); it ++) {
 		if((*it)->ctx == c && !(*it)->killed) {
-			(*it)->memory_limit = memory_limit;
+			(*it)->memory_limit = static_cast<size_t>(memory_limit);
 		}
 	}
 	V8JSG(timer_mutex).unlock();
diff --git a/v8js_class.h b/v8js_class.h
index 40c647c..6fb6f83 100644
--- a/v8js_class.h
+++ b/v8js_class.h
@@ -44,7 +44,7 @@ struct v8js_ctx {
 
   long time_limit;
   bool time_limit_hit;
-  long memory_limit;
+  size_t memory_limit;
   bool memory_limit_hit;
   long average_object_size;
 
diff --git a/v8js_timer.cc b/v8js_timer.cc
index ba2d8e1..965d865 100644
--- a/v8js_timer.cc
+++ b/v8js_timer.cc
@@ -124,7 +124,7 @@ void v8js_timer_thread(zend_v8js_globals *globals) /* {{{ */
 /* }}} */
 
 
-void v8js_timer_push(long time_limit, long memory_limit, v8js_ctx *c) /* {{{ */
+void v8js_timer_push(long time_limit, size_t memory_limit, v8js_ctx *c) /* {{{ */
 {
 	V8JSG(timer_mutex).lock();
 
diff --git a/v8js_timer.h b/v8js_timer.h
index 7cf3e6e..97abf09 100644
--- a/v8js_timer.h
+++ b/v8js_timer.h
@@ -18,14 +18,14 @@
 struct v8js_timer_ctx
 {
   long time_limit;
-  long memory_limit;
+  size_t memory_limit;
   std::chrono::time_point<std::chrono::high_resolution_clock> time_point;
   v8js_ctx *ctx;
   bool killed;
 };
 
 void v8js_timer_thread(zend_v8js_globals *globals);
-void v8js_timer_push(long time_limit, long memory_limit, v8js_ctx *c);
+void v8js_timer_push(long time_limit, size_t memory_limit, v8js_ctx *c);
 
 #endif /* V8JS_TIMER_H */
 
diff --git a/v8js_v8.cc b/v8js_v8.cc
index 9f6293d..2114d38 100644
--- a/v8js_v8.cc
+++ b/v8js_v8.cc
@@ -95,7 +95,7 @@ void v8js_v8_init() /* {{{ */
  * heap allocated memory).
  */
 void v8js_v8_call(v8js_ctx *c, zval **return_value,
-				  long flags, long time_limit, long memory_limit,
+				  long flags, long time_limit, size_t memory_limit,
 				  std::function< v8::Local<v8::Value>(v8::Isolate *) >& v8_call) /* {{{ */
 {
 	char *tz = NULL;
diff --git a/v8js_v8.h b/v8js_v8.h
index d60b6ad..e2475b0 100644
--- a/v8js_v8.h
+++ b/v8js_v8.h
@@ -53,7 +53,7 @@ static inline const char * ToCString(const v8::String::Utf8Value &value) /* {{{
 
 void v8js_v8_init();
 void v8js_v8_call(v8js_ctx *c, zval **return_value,
-				  long flags, long time_limit, long memory_limit,
+				  long flags, long time_limit, size_t memory_limit,
 				  std::function< v8::Local<v8::Value>(v8::Isolate *) >& v8_call);
 void v8js_terminate_execution(v8::Isolate *isolate);