diff --git a/openflexure_microscope/camera/mock.py b/openflexure_microscope/camera/mock.py
index 7aca48d8..04d40529 100644
--- a/openflexure_microscope/camera/mock.py
+++ b/openflexure_microscope/camera/mock.py
@@ -195,7 +195,8 @@ class MockStreamer(BaseCamera):
 
             output.write(self.stream.getvalue())
 
-            output.close()
+            if isinstance(output, str):
+                output.close()
 
     # HANDLE STREAM FRAMES