diff --git a/gprMax/grid.py b/gprMax/grid.py
index b67f0bbb..550e0581 100644
--- a/gprMax/grid.py
+++ b/gprMax/grid.py
@@ -91,6 +91,7 @@ class FDTDGrid(Grid):
         self.dy = 0
         self.dz = 0
         self.dt = 0
+        self.dimension = None
         self.iterations = 0
         self.timewindow = 0
         self.nthreads = 0