diff --git a/tests/jfjoch_hdf5_enospc_test.cpp b/tests/jfjoch_hdf5_enospc_test.cpp
index a8382f7f..c11383ed 100644
--- a/tests/jfjoch_hdf5_enospc_test.cpp
+++ b/tests/jfjoch_hdf5_enospc_test.cpp
@@ -33,7 +33,10 @@ TEST_CASE("FileWriter_enospc") {
         x.FillMessage(start_message);
 
         FileWriter file_set(start_message);
-        std::vector<uint16_t> image(x.GetPixelsNum());
+        // int16, because a JUNGFRAU in photon-counting conversion is signed by default and the
+        // writer refuses a stream whose images do not match the format its start message declares.
+        // Nothing here reads a pixel value back - the test is about running out of space.
+        std::vector<int16_t> image(x.GetPixelsNum());
 
         DataMessage message{};
         message.image = CompressedImage(image,  x.GetXPixelsNum(), x.GetYPixelsNum());