diff --git a/dlls/msacm/driver.c b/dlls/msacm/driver.c
index 808a36d28ff..bfcc533d8cc 100644
--- a/dlls/msacm/driver.c
+++ b/dlls/msacm/driver.c
@@ -184,7 +184,8 @@ MMRESULT WINAPI acmDriverDetailsA(HACMDRIVERID hadid, PACMDRIVERDETAILSA padd, D
                              sizeof(padda.szLicensing), NULL, NULL );
         WideCharToMultiByte( CP_ACP, 0, addw.szFeatures, -1, padda.szFeatures,
                              sizeof(padda.szFeatures), NULL, NULL );
-        memcpy(padd, &padda, min(padd->cbStruct, sizeof(*padd)));
+        padda.cbStruct = min(padd->cbStruct, sizeof(*padd));
+        memcpy(padd, &padda, padda.cbStruct);
     }
     return mmr;
 }
@@ -217,10 +218,12 @@ MMRESULT WINAPI acmDriverDetailsW(HACMDRIVERID hadid, PACMDRIVERDETAILSW padd, D
     mmr = acmDriverOpen(&acmDrvr, hadid, 0);
     if (mmr == MMSYSERR_NOERROR) {
         ACMDRIVERDETAILSW paddw;
+        paddw.cbStruct = sizeof(paddw);
         mmr = (MMRESULT)MSACM_Message(acmDrvr, ACMDM_DRIVER_DETAILS, (LPARAM)&paddw,  0);
 
 	acmDriverClose(acmDrvr, 0);
-        memcpy(padd, &paddw, min(padd->cbStruct, sizeof(*padd)));
+        paddw.cbStruct = min(padd->cbStruct, sizeof(*padd));
+        memcpy(padd, &paddw, paddw.cbStruct);
     }
 
     return mmr;
diff --git a/dlls/msacm/tests/msacm.c b/dlls/msacm/tests/msacm.c
index d3abae63a61..1d585fd1fc6 100644
--- a/dlls/msacm/tests/msacm.c
+++ b/dlls/msacm/tests/msacm.c
@@ -120,6 +120,15 @@ static BOOL CALLBACK DriverEnumProc(HACMDRIVERID hadid,
        "acmDriverDetails(): rc = %08x, should be %08x\n",
        rc, MMSYSERR_NOERROR);
 
+    /* cbStruct should contain size of returned data (at most sizeof(dd)) 
+       TODO: should it be *exactly* sizeof(dd), as tested here?
+     */
+    if (rc == MMSYSERR_NOERROR) {    
+        ok(dd.cbStruct == sizeof(dd),
+            "acmDriverDetails(): cbStruct = %08lx, should be %08lx\n",
+            dd.cbStruct, (unsigned long)sizeof(dd));
+    }
+
     if (rc == MMSYSERR_NOERROR && winetest_interactive) {
         trace("  Short name: %s\n", dd.szShortName);
         trace("  Long name: %s\n", dd.szLongName);