diff --git a/src/clib/CMakeLists.txt b/src/clib/CMakeLists.txt
index da2ac0f515d..3c8be4349b6 100644
--- a/src/clib/CMakeLists.txt
+++ b/src/clib/CMakeLists.txt
@@ -157,7 +157,7 @@ check_type_size("size_t" SIZEOF_SIZE_T)
 MESSAGE("size_t ${SIZEOF_SIZE_T}")
 CHECK_TYPE_SIZE("long long" SIZEOF_LONG_LONG)
 MESSAGE("long long ${SIZEOF_LONG_LONG}")
-if (NOT ${SIZEOF_SIZE_T} EQUAL ${SIZEOF_LONG_LONG})
+if (SIZEOF_SIZE_T AND SIZEOF_LONG_LONG AND NOT ${SIZEOF_SIZE_T} EQUAL ${SIZEOF_LONG_LONG})
   message (FATAL_ERROR "size_t and long long must be the same size!")
 endif ()