diff --git a/README b/README
index 5958d89..e0ace97 100644
--- a/README
+++ b/README
@@ -65,9 +65,10 @@ Most functions take a pointer to a struct sp_port, which represents an open
 port. This structure should be allocated by the user and is populated by
 sp_open(). It can be freed safely after sp_close().
 
-All functions can return only two possible error values. SP_ERR_ARG indicates
+All functions can return only three possible error values. SP_ERR_ARG indicates
 the function was called with invalid arguments. SP_ERR_FAIL indicates that the
-OS reported a failure. Both these error values are negative.
+OS reported a failure. SP_ERR_MEM indicates that a memory allocation failed.
+Aoth these error values are negative.
 
 When SP_ERR_FAIL is returned, an error code or string description of the error
 can be obtained by calling sp_last_error_code() or sp_last_error_message(). The
diff --git a/serialport.h b/serialport.h
index dafe99b..3d6410e 100644
--- a/serialport.h
+++ b/serialport.h
@@ -41,10 +41,12 @@ struct sp_port {
 enum {
 	/* Operation completed successfully. */
 	SP_OK = 0,
-	/* A system error occured while executing the operation. */
-	SP_ERR_FAIL = -1,
 	/* Invalid arguments were passed to the function. */
-	SP_ERR_ARG = -2
+	SP_ERR_ARG = -1,
+	/* A system error occured while executing the operation. */
+	SP_ERR_FAIL = -2,
+	/* A memory allocation failed while executing the operation. */
+	SP_ERR_MEM = -3
 };
 
 /* Port access modes. */