diff --git a/newsfragments/490.bugfix b/newsfragments/490.bugfix
new file mode 100644
index 000000000..6879b2042
--- /dev/null
+++ b/newsfragments/490.bugfix
@@ -0,0 +1 @@
+Fix wavelength bug in FormatXTC for older datasets
diff --git a/src/dxtbx/format/FormatXTC.py b/src/dxtbx/format/FormatXTC.py
index 00a389b75..8503dbb90 100644
--- a/src/dxtbx/format/FormatXTC.py
+++ b/src/dxtbx/format/FormatXTC.py
@@ -49,7 +49,7 @@
   use_ffb = False
     .type = bool
     .help = Run on the ffb if possible. Only for active users!
-  wavelength_delta_k = None
+  wavelength_delta_k = 0
     .type = float
     .help = Correction factor, needed during 2014
   wavelength_offset = None