diff --git a/pgtype/pgtype.go b/pgtype/pgtype.go
index 8c743163..5c9e5796 100644
--- a/pgtype/pgtype.go
+++ b/pgtype/pgtype.go
@@ -422,6 +422,8 @@ func (m *Map) TypeForValue(v interface{}) (*Type, bool) {
 	return dt, ok
 }
 
+// FormatCodeForOID returns the preferred format code for type oid. If the type is not registered it returns the text
+// format code.
 func (m *Map) FormatCodeForOID(oid uint32) int16 {
 	fc, ok := m.oidToFormatCode[oid]
 	if ok {