diff --git a/dlls/imaadp32.acm/imaadp32.c b/dlls/imaadp32.acm/imaadp32.c
index f784f59f164..dd3cc5d108e 100644
--- a/dlls/imaadp32.acm/imaadp32.c
+++ b/dlls/imaadp32.acm/imaadp32.c
@@ -688,7 +688,7 @@ static	LRESULT	ADPCM_StreamOpen(PACMDRVSTREAMINSTANCE adsi)
     aad = HeapAlloc(GetProcessHeap(), 0, sizeof(AcmAdpcmData));
     if (aad == 0) return MMSYSERR_NOMEM;
 
-    adsi->dwDriver = (DWORD)aad;
+    adsi->dwDriver = (DWORD_PTR)aad;
 
     if (adsi->pwfxSrc->wFormatTag == WAVE_FORMAT_PCM &&
 	adsi->pwfxDst->wFormatTag == WAVE_FORMAT_PCM)