diff --git a/fstest/mockobject/mockobject.go b/fstest/mockobject/mockobject.go
index 3f55e4d7c..a444d0617 100644
--- a/fstest/mockobject/mockobject.go
+++ b/fstest/mockobject/mockobject.go
@@ -96,9 +96,10 @@ var SeekModes = []SeekMode{SeekModeNone, SeekModeRegular, SeekModeRange}
 // ContentMockObject mocks an fs.Object and has content
 type ContentMockObject struct {
 	Object
-	content  []byte
-	seekMode SeekMode
-	f        fs.Fs
+	content     []byte
+	seekMode    SeekMode
+	f           fs.Fs
+	unknownSize bool
 }
 
 // WithContent returns a fs.Object with the given content.
@@ -115,6 +116,11 @@ func (o *ContentMockObject) SetFs(f fs.Fs) {
 	o.f = f
 }
 
+// SetUnknownSize makes the mock object return -1 for size if true
+func (o *ContentMockObject) SetUnknownSize(unknownSize bool) {
+	o.unknownSize = true
+}
+
 // Fs returns read only access to the Fs that this object is part of
 //
 // This is nil unless SetFs has been called
@@ -124,21 +130,22 @@ func (o *ContentMockObject) Fs() fs.Info {
 
 // Open opens the file for read.  Call Close() on the returned io.ReadCloser
 func (o *ContentMockObject) Open(ctx context.Context, options ...fs.OpenOption) (io.ReadCloser, error) {
+	size := int64(len(o.content))
 	var offset, limit int64 = 0, -1
 	for _, option := range options {
 		switch x := option.(type) {
 		case *fs.SeekOption:
 			offset = x.Offset
 		case *fs.RangeOption:
-			offset, limit = x.Decode(o.Size())
+			offset, limit = x.Decode(size)
 		default:
 			if option.Mandatory() {
 				return nil, fmt.Errorf("Unsupported mandatory option: %v", option)
 			}
 		}
 	}
-	if limit == -1 || offset+limit > o.Size() {
-		limit = o.Size() - offset
+	if limit == -1 || offset+limit > size {
+		limit = size - offset
 	}
 
 	var r *bytes.Reader
@@ -165,6 +172,9 @@ func (o *ContentMockObject) Open(ctx context.Context, options ...fs.OpenOption)
 
 // Size returns the size of the file
 func (o *ContentMockObject) Size() int64 {
+	if o.unknownSize {
+		return -1
+	}
 	return int64(len(o.content))
 }