From bf2b44737fbcb5bc09a9f10933d556dd3bd74e20 Mon Sep 17 00:00:00 2001
From: Charlie Tonneslan <cst0520@gmail.com>
Date: Tue, 12 May 2026 08:42:28 -0400
Subject: [PATCH] null: fix NullUUID.Scan returning Valid=true for empty
 string/bytes

UUID.Scan treats empty string and empty []byte as a null UUID, returning
nil without parsing. NullUUID.Scan saw that nil error and set Valid=true,
so callers would get a non-null NullUUID with the zero UUID value.

Fix by checking for empty string/bytes before delegating to UUID.Scan,
mirroring the same logic, so Valid=false is set for empty input just as
it is for an actual nil value.

Fixes #109
---
 null.go      | 16 ++++++++++++++++
 null_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/null.go b/null.go
index d7fcbf2..2e52171 100644
--- a/null.go
+++ b/null.go
@@ -38,6 +38,22 @@ func (nu *NullUUID) Scan(value interface{}) error {
 		return nil
 	}
 
+	// UUID.Scan treats empty string and empty []byte as a null UUID (returning
+	// no error). Mirror that behavior here so NullUUID.Valid reflects the same
+	// null semantics instead of incorrectly reporting Valid=true for empty input.
+	switch v := value.(type) {
+	case string:
+		if v == "" {
+			nu.UUID, nu.Valid = Nil, false
+			return nil
+		}
+	case []byte:
+		if len(v) == 0 {
+			nu.UUID, nu.Valid = Nil, false
+			return nil
+		}
+	}
+
 	err := nu.UUID.Scan(value)
 	if err != nil {
 		nu.Valid = false
diff --git a/null_test.go b/null_test.go
index fe0fe8d..4685d31 100644
--- a/null_test.go
+++ b/null_test.go
@@ -38,6 +38,32 @@ func TestNullUUIDScan(t *testing.T) {
 	}
 }
 
+func TestNullUUIDScanEmptyIsNull(t *testing.T) {
+	// An empty string or empty []byte should be treated as NULL (Valid=false),
+	// consistent with how UUID.Scan handles empty input. See issue #109.
+	var nu NullUUID
+
+	if err := nu.Scan(""); err != nil {
+		t.Fatalf("unexpected error scanning empty string: %v", err)
+	}
+	if nu.Valid {
+		t.Error("expected Valid=false after scanning empty string, got true")
+	}
+	if nu.UUID != Nil {
+		t.Errorf("expected Nil UUID after scanning empty string, got %v", nu.UUID)
+	}
+
+	if err := nu.Scan([]byte{}); err != nil {
+		t.Fatalf("unexpected error scanning empty []byte: %v", err)
+	}
+	if nu.Valid {
+		t.Error("expected Valid=false after scanning empty []byte, got true")
+	}
+	if nu.UUID != Nil {
+		t.Errorf("expected Nil UUID after scanning empty []byte, got %v", nu.UUID)
+	}
+}
+
 func TestNullUUIDValue(t *testing.T) {
 	var u UUID
 	var nu NullUUID