diff --git a/headers_test.go b/headers_test.go
index 801a4de..f0f6e11 100644
--- a/headers_test.go
+++ b/headers_test.go
@@ -133,6 +133,34 @@ func TestProtectedHeader_MarshalCBOR(t *testing.T) {
 			},
 			wantErr: true,
 		},
+		{
+			name: "content type is string",
+			h: ProtectedHeader{
+				HeaderLabelContentType: []byte("foo"),
+			},
+			wantErr: true,
+		},
+		{
+			name: "content type is negative int8",
+			h: ProtectedHeader{
+				HeaderLabelContentType: int8(-1),
+			},
+			wantErr: true,
+		},
+		{
+			name: "content type is negative int16",
+			h: ProtectedHeader{
+				HeaderLabelContentType: int16(-1),
+			},
+			wantErr: true,
+		},
+		{
+			name: "content type is negative int32",
+			h: ProtectedHeader{
+				HeaderLabelContentType: int32(-1),
+			},
+			wantErr: true,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -800,7 +828,7 @@ func TestHeaders_MarshalUnprotected(t *testing.T) {
 					HeaderLabelAlgorithm: AlgorithmES256,
 				},
 				Unprotected: UnprotectedHeader{
-					HeaderLabelContentType: 42,
+					HeaderLabelContentType: uint8(42),
 				},
 			},
 			want: []byte{0xa1, 0x03, 0x18, 0x2a},
@@ -866,7 +894,7 @@ func TestHeaders_UnmarshalFromRaw(t *testing.T) {
 				},
 				RawUnprotected: []byte{0xa1, 0x04, 0x18, 0x2a},
 				Unprotected: UnprotectedHeader{
-					HeaderLabelContentType: 42,
+					HeaderLabelContentType: int8(42),
 				},
 			},
 		},
@@ -879,7 +907,7 @@ func TestHeaders_UnmarshalFromRaw(t *testing.T) {
 				},
 				RawUnprotected: []byte{0xa1, 0x03, 0x18, 0x2a},
 				Unprotected: UnprotectedHeader{
-					HeaderLabelContentType: 43,
+					HeaderLabelContentType: int16(43),
 				},
 			},
 			want: Headers{