diff --git a/newsfragments/785.bugfix b/newsfragments/785.bugfix
new file mode 100644
index 000000000..adf4005c9
--- /dev/null
+++ b/newsfragments/785.bugfix
@@ -0,0 +1 @@
+``NXmx``: require existence of /entry/data/data virtual data set to support NXmx data, fall back to FormatNexus if missing (for e.g. DECTRIS file writer data)
diff --git a/src/dxtbx/format/FormatNXmx.py b/src/dxtbx/format/FormatNXmx.py
index 6963d5f5f..e06230009 100644
--- a/src/dxtbx/format/FormatNXmx.py
+++ b/src/dxtbx/format/FormatNXmx.py
@@ -74,13 +74,16 @@ class FormatNXmx(FormatNexus):
     @staticmethod
     def understand(image_file):
         with h5py.File(image_file) as handle:
-            return bool(
-                [
-                    entry
-                    for entry in nxmx.find_class(handle, "NXentry")
-                    if "definition" in entry
-                    and nxmx.h5str(entry["definition"][()]) == "NXmx"
-                ]
+            return (
+                bool(
+                    [
+                        entry
+                        for entry in nxmx.find_class(handle, "NXentry")
+                        if "definition" in entry
+                        and nxmx.h5str(entry["definition"][()]) == "NXmx"
+                    ]
+                )
+                and "/entry/data/data" in handle
             )
 
     def __init__(self, image_file, **kwargs):