diff --git a/types/retcode.go b/types/retcode.go
index bccd9a4c..640f6236 100644
--- a/types/retcode.go
+++ b/types/retcode.go
@@ -1,12 +1,31 @@
 package types
 
+import (
+	"errors"
+)
+
 type RetCode int
 
 // Reserved return codes
 const (
-	RetCodeOK               = RetCode(0)
-	RetCodeInternalError    = RetCode(1)
-	RetCodeUnauthorized     = RetCode(2)
-	RetCodeInsufficientFees = RetCode(3)
-	RetCodeUnknownRequest   = RetCode(4)
+	RetCodeOK               RetCode = 0
+	RetCodeInternalError    RetCode = 1
+	RetCodeUnauthorized     RetCode = 2
+	RetCodeInsufficientFees RetCode = 3
+	RetCodeUnknownRequest   RetCode = 4
 )
+
+func (r RetCode) Error() error {
+	switch r {
+	case RetCodeOK:
+		return nil
+	default:
+		return errors.New(r.String())
+	}
+}
+
+//go:generate stringer -type=RetCode
+
+// NOTE: The previous comment generates r.String().
+// To run it, `go get golang.org/x/tools/cmd/stringer`
+// and `go generate` in tmsp/types
diff --git a/types/retcode_string.go b/types/retcode_string.go
new file mode 100644
index 00000000..17ea7766
--- /dev/null
+++ b/types/retcode_string.go
@@ -0,0 +1,16 @@
+// generated by stringer -type=RetCode; DO NOT EDIT
+
+package types
+
+import "fmt"
+
+const _RetCode_name = "RetCodeOKRetCodeInternalErrorRetCodeUnauthorizedRetCodeInsufficientFeesRetCodeUnknownRequest"
+
+var _RetCode_index = [...]uint8{0, 9, 29, 48, 71, 92}
+
+func (i RetCode) String() string {
+	if i < 0 || i+1 >= RetCode(len(_RetCode_index)) {
+		return fmt.Sprintf("RetCode(%d)", i)
+	}
+	return _RetCode_name[_RetCode_index[i]:_RetCode_index[i+1]]
+}