diff --git a/gprMax/input_cmds_multiuse.py b/gprMax/input_cmds_multiuse.py
index b212d281..bd5d9b3f 100644
--- a/gprMax/input_cmds_multiuse.py
+++ b/gprMax/input_cmds_multiuse.py
@@ -275,7 +275,7 @@ def process_multicmds(multicmds, G):
 
             if xcoord < G.pmlthickness[0] or xcoord > G.nx - G.pmlthickness[3] or ycoord < G.pmlthickness[1] or ycoord > G.ny - G.pmlthickness[4] or zcoord < G.pmlthickness[2] or zcoord > G.nz - G.pmlthickness[5]:
                 print("WARNING: '" + cmdname + ': ' + ' '.join(tmp) + "'" + ' sources and receivers should not normally be positioned within the PML.\n')
-            if resistance <= 0 or resistance > z0:
+            if resistance <= 0 or resistance >= z0:
                 raise CmdInputError("'" + cmdname + ': ' + ' '.join(tmp) + "'" + ' requires a resistance greater than zero and less than the impedance of free space, i.e. 376.73 Ohms')
 
             # Check if there is a waveformID in the waveforms list