diff --git a/utils/python/CIME/case.py b/utils/python/CIME/case.py
index 3208b0b6b0ec..5be8fc921de0 100644
--- a/utils/python/CIME/case.py
+++ b/utils/python/CIME/case.py
@@ -710,8 +710,11 @@ def _create_caseroot_tools(self):
 
         # Create Macros file.
         machine = self.get_value("MACH")
-        if os.getenv("CIME_USE_CONFIG_BUILD") == "TRUE":
-            files = Files()
+        files = Files()
+        # Use config_build if the environment variable is set, or if there is no
+        # config_compilers file.
+        if os.getenv("CIME_USE_CONFIG_BUILD") == "TRUE" or \
+           files.get_value("COMPILERS_SPEC_FILE") is None:
             build_file = files.get_value("BUILD_SPEC_FILE")
             machobj = Machines(machine=machine, files=files)
             macro_maker = Build(machobj)