diff --git a/dlls/msacm32.drv/wavemap.c b/dlls/msacm32.drv/wavemap.c
index d2738aa6df6..33e328dc615 100644
--- a/dlls/msacm32.drv/wavemap.c
+++ b/dlls/msacm32.drv/wavemap.c
@@ -67,7 +67,7 @@ typedef	struct tagWAVEMAPDATA {
     DWORD       nSamplesPerSecInner;
 } WAVEMAPDATA;
 
-static	BOOL	WAVEMAP_IsData(WAVEMAPDATA* wm)
+static	BOOL	WAVEMAP_IsData(const WAVEMAPDATA* wm)
 {
     return (!IsBadReadPtr(wm, sizeof(WAVEMAPDATA)) && wm->self == wm);
 }