diff --git a/libclamunrar/CMakeLists.txt b/libclamunrar/CMakeLists.txt
index f7ce3848e..5bae50aca 100644
--- a/libclamunrar/CMakeLists.txt
+++ b/libclamunrar/CMakeLists.txt
@@ -85,6 +85,7 @@ target_sources( clamunrar
         "dll.hpp" )
 if(WIN32)
     set_target_properties(clamunrar PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS ON)
+    target_sources( clamunrar PRIVATE "isnt.cpp")
 endif()
 
 # Public (forwarded) dependencies.