diff --git a/API/main.py b/API/main.py
index a99426b..1e8d3ea 100755
--- a/API/main.py
+++ b/API/main.py
@@ -18,6 +18,7 @@ def display_text():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# receive client data
 		data = request.form
@@ -43,6 +44,7 @@ def set_pixel():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# receive client data
 		data = request.form
@@ -67,6 +69,7 @@ def draw_circle():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# receive client data
 		data = request.form
@@ -92,6 +95,7 @@ def draw_rectangle():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# receive client data
 		data = request.form
@@ -119,6 +123,7 @@ def set_color():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# receive client data
 		data = request.form
@@ -144,6 +149,7 @@ def clear_canvas():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# clear matrix canvas
 		matrix.clear()
@@ -152,6 +158,7 @@ def clear_canvas():
 		print(e)
 		response['success'] = False
 
+	# respond to client
 	return jsonify(response)
 
 @api.route('/update', methods=['GET'])
@@ -160,6 +167,7 @@ def update_matrix():
 	response = { 'success': True }
 	try:
 		global matrix
+		assert matrix is not None
 
 		# swap matrix canvas
 		matrix.update()