diff --git a/src/ess/nmx/scaling.py b/src/ess/nmx/scaling.py
index 837ebcb..c76ae0d 100644
--- a/src/ess/nmx/scaling.py
+++ b/src/ess/nmx/scaling.py
@@ -359,6 +359,10 @@ def polyval_wavelength(
 
 
     """
+    # We need to use float64 precision because
+    # the curve fit routine depends on finite difference
+    # estimates of the derivative.
+    wavelength = wavelength.to(dtype='float64')
     out = sc.zeros_like(wavelength)
     out.unit = out_unit
     xk = sc.ones_like(wavelength)