diff --git a/cmake/system.cmake b/cmake/system.cmake
index 064e7e4f2..3fdd9390c 100644
--- a/cmake/system.cmake
+++ b/cmake/system.cmake
@@ -97,7 +97,7 @@ if (NOT CMAKE_CROSSCOMPILING)
 endif()
 
 if (NOT DEFINED NUM_THREADS)
-  if (NOT NUM_CORES EQUAL 0)
+  if (DEFINED NUM_CORES AND NOT NUM_CORES EQUAL 0)
     # HT?
     set(NUM_THREADS ${NUM_CORES})
   else ()