diff --git a/filterpy/gh/gh_filter.py b/filterpy/gh/gh_filter.py
index 5561c944..a526e9a3 100644
--- a/filterpy/gh/gh_filter.py
+++ b/filterpy/gh/gh_filter.py
@@ -147,7 +147,7 @@ def update(self, z, g=None, h=None, k=None):
 
             self.x[0] = x + dxdt + 0.5*ddx*T2 + g*self.residual
             self.x[1] = dx + ddx*self.dt      + h*self.residual / self.dt
-            self.x[2] = ddx            + 2*self.k*self.residual / (self.dt**2)
+            self.x[2] = ddx                 + 2*k*self.residual / (self.dt**2)
 
 
 class GHFilter(object):