diff --git a/errors.go b/errors.go
index 45d3394..ce12733 100644
--- a/errors.go
+++ b/errors.go
@@ -2,6 +2,7 @@ package gotado
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"strings"
@@ -29,6 +30,9 @@ func (e *apiError) Error() string {
 }
 
 func isError(resp *http.Response) error {
+	if resp == nil {
+		return errors.New("response is nil")
+	}
 	if resp.StatusCode >= 200 && resp.StatusCode < 300 {
 		return nil
 	}
diff --git a/errors_test.go b/errors_test.go
new file mode 100644
index 0000000..241bad1
--- /dev/null
+++ b/errors_test.go
@@ -0,0 +1,58 @@
+package gotado
+
+import (
+	"fmt"
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIsError(t *testing.T) {
+	tests := map[string]struct {
+		resp    *http.Response
+		wantErr error
+	}{
+		"NoError": {
+			resp:    makeResponse(200, ""),
+			wantErr: nil,
+		},
+		"NilError": {
+			resp:    nil,
+			wantErr: fmt.Errorf("response is nil"),
+		},
+
+		"NoJsonError": {
+			resp:    makeResponse(404, "not found"),
+			wantErr: fmt.Errorf("unable to decode API error: invalid character 'o' in literal null (expecting 'u')"),
+		},
+		"InvalidJsonError": {
+			resp:    makeResponse(301, `{"foo": "bar"}`),
+			wantErr: fmt.Errorf("API returned empty error"),
+		},
+		"EmptyError": {
+			resp:    makeResponse(500, `{"errors":[]}`),
+			wantErr: fmt.Errorf("API returned empty error"),
+		},
+		"SingleError": {
+			resp:    makeResponse(500, `{"errors":[{"code":"1","title":"One"}]}`),
+			wantErr: fmt.Errorf("1: One"),
+		},
+		"MultiError": {
+			resp:    makeResponse(500, `{"errors":[{"code":"1","title":"One"},{"code":"2","title":"Two"}]}`),
+			wantErr: fmt.Errorf("1: One, 2: Two"),
+		},
+	}
+
+	for name, tc := range tests {
+		t.Run(name, func(t *testing.T) {
+			err := isError(tc.resp)
+
+			if tc.wantErr != nil {
+				assert.EqualError(t, err, tc.wantErr.Error())
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}