diff --git a/Modules/CheckPrototypeDefinition.cmake b/Modules/CheckPrototypeDefinition.cmake
index dfa54d8ec..e203d4c3f 100644
--- a/Modules/CheckPrototypeDefinition.cmake
+++ b/Modules/CheckPrototypeDefinition.cmake
@@ -57,7 +57,7 @@ get_filename_component(__check_proto_def_dir "${CMAKE_CURRENT_LIST_FILE}" PATH)
 
 function(CHECK_PROTOTYPE_DEFINITION _FUNCTION _PROTOTYPE _RETURN _HEADER _VARIABLE)
 
-  if ("${_VARIABLE}" MATCHES "^${_VARIABLE}$")
+  if (NOT DEFINED ${_VARIABLE})
     set(CHECK_PROTOTYPE_DEFINITION_CONTENT "/* */\n")
 
     set(CHECK_PROTOTYPE_DEFINITION_FLAGS ${CMAKE_REQUIRED_FLAGS})