diff --git a/CIME/case/case.py b/CIME/case/case.py
index 2f2d44aaca3..6f9082b0aa4 100644
--- a/CIME/case/case.py
+++ b/CIME/case/case.py
@@ -1234,7 +1234,7 @@ def _setup_mach_pes(self, pecount, multi_driver, ninst, machine_name, mpilib):
                     and value.endswith("'")
                 ):
                     value = value[1:-1]
-                if append[key]:
+                if key in append and append[key]:
                     ovalue = self.get_value(key)
 
                 self.set_value(key, value + " " + ovalue)