diff --git a/tests/server/CMakeLists.txt b/tests/server/CMakeLists.txt
index 3d74519ff6..5875027767 100644
--- a/tests/server/CMakeLists.txt
+++ b/tests/server/CMakeLists.txt
@@ -48,7 +48,7 @@ target_include_directories(${BUNDLE} PRIVATE
 # to build the servers.  In order to achieve proper linkage of these
 # files on Windows targets it is necessary to build the test servers
 # with CURL_STATICLIB defined, independently of how libcurl is built.
-target_compile_definitions(${BUNDLE} PRIVATE "WITHOUT_LIBCURL" "CURL_NO_OLDIES" "$<$<BOOL:WIN32>:CURL_STATICLIB>")
+target_compile_definitions(${BUNDLE} PRIVATE "WITHOUT_LIBCURL" "CURL_NO_OLDIES" "$<$<BOOL:${WIN32}>:CURL_STATICLIB>")
 set_target_properties(${BUNDLE} PROPERTIES OUTPUT_NAME "${BUNDLE}" PROJECT_LABEL "Test ${BUNDLE}" UNITY_BUILD OFF C_CLANG_TIDY "")
 
 curl_add_clang_tidy_test_target("${BUNDLE}-clang-tidy" ${BUNDLE} ${FIRST_C} ${UTILS_C} ${TESTS_C})