diff --git a/sounddevice_build.py b/sounddevice_build.py
index 290dcfd..70ecbd8 100644
--- a/sounddevice_build.py
+++ b/sounddevice_build.py
@@ -307,6 +307,8 @@
     PaWasapiStreamCategory streamCategory;
     PaWasapiStreamOption streamOption;
 } PaWasapiStreamInfo;
+
+int PaWasapi_IsLoopback( PaDeviceIndex device );
 """)
 
 ffibuilder.cdef("""