diff --git a/common/misc/hmp.cpp b/common/misc/hmp.cpp
index 683d3c390..f0b2276ef 100644
--- a/common/misc/hmp.cpp
+++ b/common/misc/hmp.cpp
@@ -458,7 +458,7 @@ int hmp_play(hmp_file *hmp, int bLoop)
 
 	if ((rc = setup_buffers(hmp)))
 		return rc;
-	if ((midiStreamOpen(&hmp->hmidi, &hmp->devid,1, (DWORD_PTR) midi_callback, 0, CALLBACK_FUNCTION)) != MMSYSERR_NOERROR) {
+	if ((midiStreamOpen(&hmp->hmidi, &hmp->devid,1, (DWORD) (size_t) midi_callback, 0, CALLBACK_FUNCTION)) != MMSYSERR_NOERROR) {
 		hmp->hmidi = NULL;
 		return HMP_MM_ERR;
 	}