diff --git a/modhex_test.go b/modhex_test.go
index edcd96f..52a79e9 100644
--- a/modhex_test.go
+++ b/modhex_test.go
@@ -87,3 +87,40 @@ func TestDecodeString(t *testing.T) {
 		}
 	}
 }
+
+type errTest struct {
+	in  string
+	out string
+	err error
+}
+
+var errTests = []errTest{
+	{"", "", nil},
+	{"c", "", ErrLength},
+	{"zteff", "", InvalidByteError('z')},
+	{"dtefz", "\x2d\x34", InvalidByteError('z')},
+	{"ecebe", "01", ErrLength},
+	{"cq", "", InvalidByteError('q')},
+	{"ccqq", "\x00", InvalidByteError('q')},
+	{"c\x01", "", InvalidByteError('\x01')},
+	{"vvuut", "\xff\xee", ErrLength},
+}
+
+func TestDecodeErr(t *testing.T) {
+	for i, test := range errTests {
+		out := make([]byte, len(test.in)+10)
+		n, err := Decode(out, []byte(test.in))
+		if string(out[:n]) != test.out || err != test.err {
+			t.Errorf("#%d: Decode(%q) = %q, %v, want %q, %v", i, test.in, string(out[:n]), err, test.out, test.err)
+		}
+	}
+}
+
+func TestDecodeStringErr(t *testing.T) {
+	for i, test := range errTests {
+		out, err := DecodeString(test.in)
+		if string(out) != test.out || err != test.err {
+			t.Errorf("#%d: DecodeString(%q) = %q, %v, want %q, %v", i, test.in, out, err, test.out, test.err)
+		}
+	}
+}