diff --git a/scripts/lib/CIME/case.py b/scripts/lib/CIME/case.py
index d450c0ff1c2..7a0b44f818e 100644
--- a/scripts/lib/CIME/case.py
+++ b/scripts/lib/CIME/case.py
@@ -1116,7 +1116,7 @@ def get_mpirun_cmd(self, job="case.run"):
             "unit_testing" : False
             }
 
-        executable, mpi_arg_string = env_mach_specific.get_mpirun(self, mpi_attribs, job=job)
+        executable, mpi_arg_list = env_mach_specific.get_mpirun(self, mpi_attribs, job=job)
 
         # special case for aprun
         if executable is not None and "aprun" in executable:
@@ -1124,7 +1124,7 @@ def get_mpirun_cmd(self, job="case.run"):
             expect(num_nodes == self.num_nodes, "Not using optimized num nodes")
             return executable + aprun_args + " " + run_misc_suffix
         else:
-            mpi_arg_string = " ".join(args.values())
+            mpi_arg_string = " ".join(mpi_arg_list)
 
         if self.get_value("BATCH_SYSTEM") == "cobalt":
             mpi_arg_string += " : "