diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3fcde11..1acc0b5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -128,7 +128,7 @@ if(MSVC)
     # handle msvcrt setting being passed in CMAKE_C_FLAGS
     if(CMAKE_C_FLAGS MATCHES "[/-]M[TD]d?")
         # ensure CMAKE_MSVC_RUNTIME_LIBRARY is not already defined
-        if(DEFINED CMAKE_MSVC_RUNTIME_LIBRARY)
+        if(NOT CMAKE_MSVC_RUNTIME_LIBRARY)
             message(FATAL_ERROR "please set the runtime library via either CMAKE_C_FLAGS or CMAKE_MSVC_RUNTIME_LIBRARY, not both")
         endif()