diff --git a/runtime/CMakeLists.txt b/runtime/CMakeLists.txt
index 39cc92a9..07b657cc 100644
--- a/runtime/CMakeLists.txt
+++ b/runtime/CMakeLists.txt
@@ -315,10 +315,12 @@ macro(build_runtime d_flags c_flags ld_flags lib_suffix path_suffix outlist_targ
         dc(${f} "${d_flags}" "${RUNTIME_DIR}" "${target_suffix}" DCRT_O DCRT_BC)
     endforeach()
 
-    # Always build zlib and other C parts of the runtime in release mode.
-    set_source_files_properties(${CORE_C} ${DCRT_C} PROPERTIES
-        COMPILE_FLAGS "${CMAKE_C_FLAGS} ${CMAKE_C_FLAGS_RELEASE}"
-    )
+    if(NOT MSVC)
+        # Always build zlib and other C parts of the runtime in release mode.
+        set_source_files_properties(${CORE_C} ${DCRT_C} PROPERTIES
+            COMPILE_FLAGS "${CMAKE_C_FLAGS} ${CMAKE_C_FLAGS_RELEASE}"
+        )
+    endif()
 
     if(EXISTS ${RUNTIME_DIR})
         set(GCCBUILTINS "${PROJECT_BINARY_DIR}/gccbuiltins_x86.di")