diff --git a/httplib.go b/httplib.go
index e820f41..8f85c0b 100644
--- a/httplib.go
+++ b/httplib.go
@@ -107,12 +107,11 @@ func unmarshalError(err error, responseBody []byte) error {
 	}
 	var raw RawTrace
 	if err2 := json.Unmarshal(responseBody, &raw); err2 != nil {
-		return err
+		return errorOnInvalidJSON(err, responseBody)
 	}
 	if len(raw.Err) != 0 {
-		err2 := json.Unmarshal(raw.Err, err)
-		if err2 != nil {
-			return err
+		if err2 := json.Unmarshal(raw.Err, err); err2 != nil {
+			return errorOnInvalidJSON(err, responseBody)
 		}
 		return &TraceErr{
 			Traces:   raw.Traces,
@@ -122,6 +121,19 @@ func unmarshalError(err error, responseBody []byte) error {
 			Fields:   raw.Fields,
 		}
 	}
-	json.Unmarshal(responseBody, err)
+	if err2 := json.Unmarshal(responseBody, err); err2 != nil {
+		return errorOnInvalidJSON(err, responseBody)
+	}
 	return err
 }
+
+// errorOnInvalidJSON is used to construct a TraceErr with the
+// input error as Err and the responseBody as Messages.
+// This function is used when the responseBody is not valid
+// JSON or it contains an unexpected JSON.
+func errorOnInvalidJSON(err error, responseBody []byte) error {
+	return &TraceErr{
+		Err:      err,
+		Messages: []string{string(responseBody)},
+	}
+}
diff --git a/httplib_test.go b/httplib_test.go
index 3f12e0a..56b2865 100644
--- a/httplib_test.go
+++ b/httplib_test.go
@@ -26,27 +26,35 @@ import (
 
 func TestReplyJSON(t *testing.T) {
 	t.Parallel()
-	var (
-		errCode               = 400
-		errText               = "test error"
-		expectedErrorResponse = "" +
-			"{\n" +
-			"    \"error\": {\n" +
-			"        \"message\": \"" + errText + "\"\n" +
-			"    }\n" +
-			"}"
-	)
 
-	for _, tc := range []struct {
+	var expectedErrorResponse = `{
+		"error": {
+			"message": "test error"
+		}
+	}`
+
+	tests := []struct {
 		desc string
 		err  error
 	}{
-		{"plain error", errors.New("test error")},
-		{"trace error", &TraceErr{Err: errors.New("test error")}},
-		{"trace error with stacktrace", &TraceErr{Err: errors.New("test error"), Traces: Traces{{Path: "A", Func: "B", Line: 1}}}},
-	} {
+		{
+			desc: "plain error",
+			err:  errors.New("test error"),
+		},
+		{
+			desc: "trace error",
+			err:  &TraceErr{Err: errors.New("test error")},
+		},
+		{
+			desc: "trace error with stacktrace",
+			err:  &TraceErr{Err: errors.New("test error"), Traces: Traces{{Path: "A", Func: "B", Line: 1}}},
+		},
+	}
+
+	for _, tc := range tests {
 		t.Run(tc.desc, func(t *testing.T) {
 			recorder := httptest.NewRecorder()
+			const errCode = 400
 			replyJSON(recorder, errCode, tc.err)
 			require.JSONEq(t, expectedErrorResponse, recorder.Body.String())
 		})
@@ -55,12 +63,63 @@ func TestReplyJSON(t *testing.T) {
 
 func TestUnmarshalError(t *testing.T) {
 	t.Parallel()
-	testCase := func(t *testing.T, err error, response string, isExpectedErr func(error) bool, expectedMsg string) {
-		readErr := unmarshalError(err, []byte(response))
-		require.True(t, isExpectedErr(readErr))
-		require.EqualError(t, readErr, expectedMsg)
+
+	tests := []struct {
+		desc          string
+		inputErr      error
+		inputResponse string
+		assertErr     func(error) bool
+		expectedMsg   string
+	}{
+		{
+			desc:          "unmarshal not found error",
+			inputErr:      &NotFoundError{},
+			inputResponse: `{"error": {"message": "ABC"}}`,
+			assertErr:     IsNotFound,
+			expectedMsg:   "ABC",
+		},
+		{
+			desc:          "unmarshal access denied error",
+			inputErr:      &AccessDeniedError{},
+			inputResponse: `{"error": {"message": "ABC"}}`,
+			assertErr:     IsAccessDenied,
+			expectedMsg:   "ABC",
+		},
+		{
+			desc:          "unmarshal error without error JSON key",
+			inputErr:      &AccessDeniedError{},
+			inputResponse: `{"message": "ABC"}`,
+			assertErr:     IsAccessDenied,
+			expectedMsg:   "ABC",
+		},
+		{
+			desc:          "unmarshal invalid error",
+			inputErr:      &AccessDeniedError{},
+			inputResponse: `{"error": "message ABC"}`,
+			assertErr:     IsAccessDenied,
+			expectedMsg:   "{\"error\": \"message ABC\"}\n\taccess denied",
+		},
+		{
+			desc:          "unmarshal invalid error without error JSON key",
+			inputErr:      &AccessDeniedError{},
+			inputResponse: `["error message ABC"]`,
+			assertErr:     IsAccessDenied,
+			expectedMsg:   "[\"error message ABC\"]\n\taccess denied",
+		},
+		{
+			desc:          "unmarshal error with non-JSON body",
+			inputErr:      &AccessDeniedError{},
+			inputResponse: "error message ABC",
+			assertErr:     IsAccessDenied,
+			expectedMsg:   "error message ABC\n\taccess denied",
+		},
 	}
 
-	testCase(t, &NotFoundError{}, `{"error": {"message": "ABC"}}`, IsNotFound, "ABC")
-	testCase(t, &AccessDeniedError{}, `{"error": {"message": "ABC"}}`, IsAccessDenied, "ABC")
+	for _, tc := range tests {
+		t.Run(tc.desc, func(t *testing.T) {
+			readErr := unmarshalError(tc.inputErr, []byte(tc.inputResponse))
+			require.True(t, tc.assertErr(readErr))
+			require.EqualError(t, readErr, tc.expectedMsg)
+		})
+	}
 }