diff --git a/diagnose.go b/diagnose.go
index 40179060..8af4d7aa 100644
--- a/diagnose.go
+++ b/diagnose.go
@@ -183,7 +183,7 @@ func (di *diagnose) diag() (string, error) {
 	// CBOR Sequence
 	firstItem := true
 	for {
-		switch err := di.valid(); err {
+		switch err := di.wellformed(); err {
 		case nil:
 			if !firstItem {
 				if err = di.writeString(", "); err != nil {
@@ -204,9 +204,9 @@ func (di *diagnose) diag() (string, error) {
 	}
 }
 
-func (di *diagnose) valid() error {
+func (di *diagnose) wellformed() error {
 	off := di.d.off
-	err := di.d.valid(di.dm.cborSequence)
+	err := di.d.wellformed(di.dm.cborSequence)
 	di.d.off = off
 	return err
 }
@@ -372,11 +372,19 @@ func (di *diagnose) item() error { //nolint:gocyclo
 		_, _, tagNum := di.d.getHead()
 		switch tagNum {
 		case 2:
+			if di.d.nextCBORType() != cborTypeByteString {
+				return errors.New("cbor: tag number 2 must be followed by byte string, got " + t.String())
+			}
+
 			b := di.d.parseByteString()
 			bi := new(big.Int).SetBytes(b)
 			return di.writeString(bi.String())
 
 		case 3:
+			if di.d.nextCBORType() != cborTypeByteString {
+				return errors.New("cbor: tag number 3 must be followed by byte string, got " + t.String())
+			}
+
 			b := di.d.parseByteString()
 			bi := new(big.Int).SetBytes(b)
 			bi.Add(bi, big.NewInt(1))
diff --git a/diagnose_test.go b/diagnose_test.go
index c87b2a49..e6430e92 100644
--- a/diagnose_test.go
+++ b/diagnose_test.go
@@ -780,3 +780,76 @@ func TestDiagnoseCBORSequences(t *testing.T) {
 		})
 	}
 }
+
+func TestDiagnoseTag(t *testing.T) {
+	testCases := []struct {
+		title       string
+		cbor        []byte
+		diag        string
+		opts        *DiagOptions
+		returnError bool
+	}{
+		{
+			"CBOR tag number 2 with not well-formed encoded CBOR data item",
+			hexDecode("c201"),
+			``,
+			&DiagOptions{},
+			true,
+		},
+		{
+			"CBOR tag number 3 with not well-formed encoded CBOR data item",
+			hexDecode("c301"),
+			``,
+			&DiagOptions{},
+			true,
+		},
+		{
+			"CBOR tag number 2 with well-formed encoded CBOR data item",
+			hexDecode("c240"),
+			`0`,
+			&DiagOptions{},
+			false,
+		},
+		{
+			"CBOR tag number 3 with well-formed encoded CBOR data item",
+			hexDecode("c340"),
+			`-1`, // -1 - n
+			&DiagOptions{},
+			false,
+		},
+		{
+			"CBOR tag number 2 with well-formed encoded CBOR data item",
+			hexDecode("c249010000000000000000"),
+			`18446744073709551616`,
+			&DiagOptions{},
+			false,
+		},
+		{
+			"CBOR tag number 3 with well-formed encoded CBOR data item",
+			hexDecode("c349010000000000000000"),
+			`-18446744073709551617`, // -1 - n
+			&DiagOptions{},
+			false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.title, func(t *testing.T) {
+			dm, err := tc.opts.DiagMode()
+			if err != nil {
+				t.Errorf("DiagMode() for 0x%x returned error %q", tc.cbor, err)
+			}
+
+			str, err := dm.Diagnose(tc.cbor)
+			if tc.returnError && err == nil {
+				t.Errorf("Diagnose(0x%x) returned error %q", tc.cbor, err)
+			} else if !tc.returnError && err != nil {
+				t.Errorf("Diagnose(0x%x) returned error %q", tc.cbor, err)
+			}
+
+			if str != tc.diag {
+				t.Errorf("Diagnose(0x%x) returned `%s`, want %s", tc.cbor, str, tc.diag)
+			}
+		})
+	}
+}