diff --git a/proto/decode_test.go b/proto/decode_test.go
index e296fc2..b2a3369 100644
--- a/proto/decode_test.go
+++ b/proto/decode_test.go
@@ -134,6 +134,20 @@
 	}
 }
 
+func TestDecodeInvalidFieldNumbers(t *testing.T) {
+	for _, test := range invalidFieldNumberTestProtos {
+		t.Run(test.desc, func(t *testing.T) {
+			decoded := new(testpb.TestAllTypes) // type doesn't matter since we expect errors
+			err := proto.Unmarshal(test.wire, decoded)
+			if err == nil && !test.allowed {
+				t.Error("unmarshal: got nil want error")
+			} else if err != nil && test.allowed {
+				t.Errorf("unmarshal: got %v want nil since %s is allowed by Unmarshal", err, test.desc)
+			}
+		})
+	}
+}
+
 var testProtos = []testProto{
 	{
 		desc: "basic scalar types",
@@ -1663,6 +1677,55 @@
 	},
 }
 
+var invalidFieldNumberTestProtos = []struct {
+	desc    string
+	wire    []byte
+	allowed bool
+}{
+	{
+		desc: "zero",
+		wire: pack.Message{
+			pack.Tag{pack.MinValidNumber - 1, pack.VarintType}, pack.Varint(1001),
+		}.Marshal(),
+	},
+	{
+		desc: "zero and one",
+		wire: pack.Message{
+			pack.Tag{pack.MinValidNumber - 1, pack.VarintType}, pack.Varint(1002),
+			pack.Tag{pack.MinValidNumber, pack.VarintType}, pack.Varint(1003),
+		}.Marshal(),
+	},
+	{
+		desc: "first reserved",
+		wire: pack.Message{
+			pack.Tag{pack.FirstReservedNumber, pack.VarintType}, pack.Varint(1004),
+		}.Marshal(),
+		allowed: true,
+	},
+	{
+		desc: "last reserved",
+		wire: pack.Message{
+			pack.Tag{pack.LastReservedNumber, pack.VarintType}, pack.Varint(1005),
+		}.Marshal(),
+		allowed: true,
+	},
+	{
+		desc: "max and max+1",
+		wire: pack.Message{
+			pack.Tag{pack.MaxValidNumber, pack.VarintType}, pack.Varint(1006),
+			pack.Tag{pack.MaxValidNumber + 1, pack.VarintType}, pack.Varint(1007),
+		}.Marshal(),
+		allowed: flags.ProtoLegacy,
+	},
+	{
+		desc: "max+1",
+		wire: pack.Message{
+			pack.Tag{pack.MaxValidNumber + 1, pack.VarintType}, pack.Varint(1008),
+		}.Marshal(),
+		allowed: flags.ProtoLegacy,
+	},
+}
+
 func build(m proto.Message, opts ...buildOpt) proto.Message {
 	for _, opt := range opts {
 		opt(m)