diff --git a/src/SConscript b/src/SConscript
index 6c8ccaf030..d3c5a5cd8b 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -563,7 +563,10 @@ env.Command('config/the_isa.hh', [],
 def makeTheGPUISA(source, target, env):
     gpu_isa = env['TARGET_GPU_ISA']
 
-    namespace = gpu_isa[0].upper() + gpu_isa[1:].lower() + 'ISA'
+    if gpu_isa:
+        namespace = gpu_isa[0].upper() + gpu_isa[1:].lower() + 'ISA'
+    else:
+        namespace = 'None'
 
     code = code_formatter()
     code('''\