diff --git a/lib/mask128.rb b/lib/mask128.rb
index 6f297bd..8d24093 100755
--- a/lib/mask128.rb
+++ b/lib/mask128.rb
@@ -30,7 +30,9 @@ def Mask128.parse(mask)
 			if (mask.start_with?("/")) # cidr format
 				mask = mask[1..-1] # remove "/"
 			end
-			return Mask128.new(mask.to_i)
+			return Mask128.new(Integer(mask))
+		rescue ArgumentError
+			raise ValidationError, "#{mask} is not valid integer."
 		end
 		
 		#cmp compares equality with another Mask128. Return:
diff --git a/lib/mask32.rb b/lib/mask32.rb
index debe8e7..7cc719a 100755
--- a/lib/mask32.rb
+++ b/lib/mask32.rb
@@ -28,9 +28,15 @@ def initialize(prefix_len)
 		def Mask32.parse(mask)
 			mask.strip!
 			if (mask.start_with?("/")) # cidr format
-				return Mask32.new(mask[1..-1].to_i) # remove "/"
-			elsif (!mask.include?("."))
-				return Mask32.new(mask.to_i)
+				mask = mask[1..-1] # remove "/"
+			end
+
+			if (!mask.include?("."))
+				begin
+					return Mask32.new(Integer(mask))
+				rescue ArgumentError
+					raise ValidationError, "#{mask} is not valid integer."
+				end
 			end
 			
 			# for extended netmask