diff --git a/internal/error.go b/internal/error.go
new file mode 100644
index 0000000..537c02c
--- /dev/null
+++ b/internal/error.go
@@ -0,0 +1,9 @@
+package internal
+
+// Error is constant type error.
+type Error string
+
+// Error implementation of error interface.
+func (e Error) Error() string {
+	return string(e)
+}