diff --git a/CMakeLists.txt b/CMakeLists.txt
index 082b1d8..0cba0f0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -48,17 +48,17 @@ include(CheckFunctionExists)
 set(LIBM_LIBRARY)
 CHECK_FUNCTION_EXISTS(log2 LOG2_RES)
 if(NOT LOG2_RES)
-  unset(LOG2_RES)
   set(orig_req_libs "${CMAKE_REQUIRED_LIBRARIES}")
   set(CMAKE_REQUIRED_LIBRARIES "${CMAKE_REQUIRED_LIBRARIES};m")
-  CHECK_FUNCTION_EXISTS(log2 LOG2_RES)
-  if(LOG2_RES)
+  CHECK_FUNCTION_EXISTS(log2 LOG2_LIBM_RES)
+  if(LOG2_LIBM_RES)
     set(LIBM_LIBRARY "m")
   else()
     message(FATAL_ERROR "log2() not found")
   endif()
 
   set(CMAKE_REQUIRED_LIBRARIES "${orig_req_libs}")
+  unset(LOG2_LIBM_RES)
   unset(orig_req_libs)
 endif()
 unset(LOG2_RES)