diff --git a/CMakeLists.txt b/CMakeLists.txt
index be1961da67a..432e87ad195 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -669,7 +669,7 @@ if(MSVC)
 else()
   include(TestCXXAcceptsFlag)
   message(STATUS "Building on ${CMAKE_SYSTEM_PROCESSOR} for ${ARCH}")
-  if(ARCH STREQUAL "default")
+  if(NOT ARCH OR ARCH STREQUAL "default")
     set(ARCH_FLAG "")
   elseif(PPC64LE)
     set(ARCH_FLAG "-mcpu=power8")