diff --git a/uart.hpp b/uart.hpp
index 4839296..16451b7 100644
--- a/uart.hpp
+++ b/uart.hpp
@@ -192,7 +192,7 @@ class Uart {
 		return *this;
 	}
 
-	Uart &operator<<(unsigned short &val)
+	Uart &operator<<(const unsigned short &val)
 	{
 		txNumber(val);
 		return *this;
@@ -216,19 +216,19 @@ class Uart {
 		return *this;
 	}
 
-	Uart &operator<<(unsigned long &val)
+	Uart &operator<<(const unsigned long &val)
 	{
 		txNumber(val);
 		return *this;
 	}
 
-	Uart &operator<<(long long &val)
+	Uart &operator<<(const long long &val)
 	{
 		txNumber(val);
 		return *this;
 	}
 
-	Uart &operator<<(unsigned long long &val)
+	Uart &operator<<(const unsigned long long &val)
 	{
 		txNumber(val);
 		return *this;