diff --git a/dlls/midimap/midimap.c b/dlls/midimap/midimap.c
index a55c722f5f4..5df8472e68a 100644
--- a/dlls/midimap/midimap.c
+++ b/dlls/midimap/midimap.c
@@ -425,7 +425,7 @@ static DWORD modData(MIDIMAPDATA* mom, DWORD_PTR dwParam)
 static DWORD modPrepare(MIDIMAPDATA* mom, LPMIDIHDR lpMidiHdr, DWORD_PTR dwSize)
 {
     if (MIDIMAP_IsBadData(mom)) return MMSYSERR_ERROR;
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE))
 	return MMSYSERR_INVALPARAM;
 
diff --git a/dlls/winealsa.drv/midi.c b/dlls/winealsa.drv/midi.c
index e792c655ba2..00a54bf6bf1 100644
--- a/dlls/winealsa.drv/midi.c
+++ b/dlls/winealsa.drv/midi.c
@@ -567,7 +567,7 @@ static DWORD midAddBuffer(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
     if (MidiInDev[wDevID].state == -1) return MIDIERR_NODEVICE;
 
     if (lpMidiHdr == NULL)	return MMSYSERR_INVALPARAM;
-    if (sizeof(MIDIHDR) > dwSize) return MMSYSERR_INVALPARAM;
+    if (dwSize < offsetof(MIDIHDR,dwOffset)) return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwBufferLength == 0) return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwFlags & MHDR_INQUEUE) return MIDIERR_STILLPLAYING;
     if (!(lpMidiHdr->dwFlags & MHDR_PREPARED)) return MIDIERR_UNPREPARED;
@@ -598,7 +598,7 @@ static DWORD midPrepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
 {
     TRACE("(%04X, %p, %08X);\n", wDevID, lpMidiHdr, dwSize);
 
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE) != 0)
 	return MMSYSERR_INVALPARAM;
 
@@ -619,7 +619,7 @@ static DWORD midUnprepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
     if (wDevID >= MIDM_NumDevs) return MMSYSERR_BADDEVICEID;
     if (MidiInDev[wDevID].state == -1) return MIDIERR_NODEVICE;
 
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0)
 	return MMSYSERR_INVALPARAM;
 
@@ -1032,7 +1032,7 @@ static DWORD modPrepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
      * asks to prepare MIDIHDR which dwFlags != 0.
      * So at least check for the inqueue flag
      */
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE) != 0) {
 	WARN("%p %p %08x %d\n", lpMidiHdr, lpMidiHdr ? lpMidiHdr->lpData : NULL,
              lpMidiHdr ? lpMidiHdr->dwFlags : 0, dwSize);
@@ -1057,7 +1057,7 @@ static DWORD modUnprepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
 	return MMSYSERR_NOTENABLED;
     }
 
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0)
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0)
 	return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwFlags & MHDR_INQUEUE)
 	return MIDIERR_STILLPLAYING;
diff --git a/dlls/winecoreaudio.drv/midi.c b/dlls/winecoreaudio.drv/midi.c
index bb105fcb720..56eff5a4e82 100644
--- a/dlls/winecoreaudio.drv/midi.c
+++ b/dlls/winecoreaudio.drv/midi.c
@@ -463,10 +463,10 @@ static DWORD MIDIOut_Prepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
      * asks to prepare MIDIHDR which dwFlags != 0.
      * So at least check for the inqueue flag
      */
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE) != 0) {
 	WARN("%p %p %08x %lu/%d\n", lpMidiHdr, lpMidiHdr->lpData,
-	           lpMidiHdr->dwFlags, sizeof(MIDIHDR), dwSize);
+	           lpMidiHdr->dwFlags, offsetof(MIDIHDR,dwOffset), dwSize);
 	return MMSYSERR_INVALPARAM;
     }
 
@@ -487,7 +487,7 @@ static DWORD MIDIOut_Unprepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
         WARN("bad device ID : %d\n", wDevID);
 	return MMSYSERR_BADDEVICEID;
     }
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0)
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0)
 	return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwFlags & MHDR_INQUEUE)
 	return MIDIERR_STILLPLAYING;
@@ -666,7 +666,7 @@ static DWORD MIDIIn_AddBuffer(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
 	WARN("Invalid Parameter\n");
 	return MMSYSERR_INVALPARAM;
     }
-    if (sizeof(MIDIHDR) > dwSize) {
+    if (dwSize < offsetof(MIDIHDR,dwOffset)) {
 	WARN("Invalid Parameter\n");
 	return MMSYSERR_INVALPARAM;
     }
@@ -714,7 +714,7 @@ static DWORD MIDIIn_Prepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
      * asks to prepare MIDIHDR which dwFlags != 0.
      * So at least check for the inqueue flag
      */
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE) != 0) {
 	WARN("Invalid parameter %p %p %08x %d\n", lpMidiHdr, lpMidiHdr->lpData,
 	           lpMidiHdr->dwFlags, dwSize);
@@ -734,7 +734,7 @@ static DWORD MIDIIn_Unprepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
         WARN("bad device ID : %d\n", wDevID);
 	return MMSYSERR_BADDEVICEID;
     }
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0) {
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0) {
 	WARN("Invalid Parameter\n");
 	return MMSYSERR_INVALPARAM;
     }
diff --git a/dlls/wineoss.drv/midi.c b/dlls/wineoss.drv/midi.c
index 52a60666ee3..0945e933710 100644
--- a/dlls/wineoss.drv/midi.c
+++ b/dlls/wineoss.drv/midi.c
@@ -829,7 +829,7 @@ static DWORD midAddBuffer(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
     if (MidiInDev[wDevID].state == -1) return MIDIERR_NODEVICE;
 
     if (lpMidiHdr == NULL)	return MMSYSERR_INVALPARAM;
-    if (sizeof(MIDIHDR) > dwSize) return MMSYSERR_INVALPARAM;
+    if (dwSize < offsetof(MIDIHDR,dwOffset)) return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwBufferLength == 0) return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwFlags & MHDR_INQUEUE) return MIDIERR_STILLPLAYING;
     if (!(lpMidiHdr->dwFlags & MHDR_PREPARED)) return MIDIERR_UNPREPARED;
@@ -861,7 +861,7 @@ static DWORD midPrepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
 {
     TRACE("(%04X, %p, %08X);\n", wDevID, lpMidiHdr, dwSize);
 
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE) != 0)
 	return MMSYSERR_INVALPARAM;
 
@@ -882,7 +882,7 @@ static DWORD midUnprepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
     if (wDevID >= MIDM_NumDevs) return MMSYSERR_BADDEVICEID;
     if (MidiInDev[wDevID].state == -1) return MIDIERR_NODEVICE;
 
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0)
 	return MMSYSERR_INVALPARAM;
 
@@ -1595,7 +1595,7 @@ static DWORD modPrepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
      * asks to prepare MIDIHDR which dwFlags != 0.
      * So at least check for the inqueue flag
      */
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0 ||
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0 ||
 	lpMidiHdr->lpData == 0 || (lpMidiHdr->dwFlags & MHDR_INQUEUE) != 0) {
 	WARN("%p %p %08x %d\n", lpMidiHdr, lpMidiHdr ? lpMidiHdr->lpData : NULL,
 	           lpMidiHdr ? lpMidiHdr->dwFlags : 0, dwSize);
@@ -1620,7 +1620,7 @@ static DWORD modUnprepare(WORD wDevID, LPMIDIHDR lpMidiHdr, DWORD dwSize)
 	return MMSYSERR_NOTENABLED;
     }
 
-    if (dwSize < sizeof(MIDIHDR) || lpMidiHdr == 0)
+    if (dwSize < offsetof(MIDIHDR,dwOffset) || lpMidiHdr == 0)
 	return MMSYSERR_INVALPARAM;
     if (lpMidiHdr->dwFlags & MHDR_INQUEUE)
 	return MIDIERR_STILLPLAYING;
diff --git a/dlls/winmm/winmm.c b/dlls/winmm/winmm.c
index 06aed09587f..99e0361e6f7 100644
--- a/dlls/winmm/winmm.c
+++ b/dlls/winmm/winmm.c
@@ -964,7 +964,7 @@ UINT WINAPI midiOutPrepareHeader(HMIDIOUT hMidiOut,
 
     TRACE("(%p, %p, %d)\n", hMidiOut, lpMidiOutHdr, uSize);
 
-    if (lpMidiOutHdr == NULL || uSize < sizeof (MIDIHDR))
+    if (lpMidiOutHdr == NULL || uSize < offsetof(MIDIHDR,dwOffset))
 	return MMSYSERR_INVALPARAM;
 
     if ((wmld = MMDRV_Get(hMidiOut, MMDRV_MIDIOUT, FALSE)) == NULL)
@@ -984,7 +984,7 @@ UINT WINAPI midiOutUnprepareHeader(HMIDIOUT hMidiOut,
 
     TRACE("(%p, %p, %d)\n", hMidiOut, lpMidiOutHdr, uSize);
 
-    if (lpMidiOutHdr == NULL || uSize < sizeof (MIDIHDR))
+    if (lpMidiOutHdr == NULL || uSize < offsetof(MIDIHDR,dwOffset))
 	return MMSYSERR_INVALPARAM;
 
     if (!(lpMidiOutHdr->dwFlags & MHDR_PREPARED)) {
@@ -1258,7 +1258,7 @@ UINT WINAPI midiInPrepareHeader(HMIDIIN hMidiIn,
 
     TRACE("(%p, %p, %d)\n", hMidiIn, lpMidiInHdr, uSize);
 
-    if (lpMidiInHdr == NULL || uSize < sizeof (MIDIHDR))
+    if (lpMidiInHdr == NULL || uSize < offsetof(MIDIHDR,dwOffset))
 	return MMSYSERR_INVALPARAM;
 
     if ((wmld = MMDRV_Get(hMidiIn, MMDRV_MIDIIN, FALSE)) == NULL)
@@ -1277,7 +1277,7 @@ UINT WINAPI midiInUnprepareHeader(HMIDIIN hMidiIn,
 
     TRACE("(%p, %p, %d)\n", hMidiIn, lpMidiInHdr, uSize);
 
-    if (lpMidiInHdr == NULL || uSize < sizeof (MIDIHDR))
+    if (lpMidiInHdr == NULL || uSize < offsetof(MIDIHDR,dwOffset))
 	return MMSYSERR_INVALPARAM;
 
     if (!(lpMidiInHdr->dwFlags & MHDR_PREPARED)) {
@@ -1819,7 +1819,7 @@ MMRESULT WINAPI midiStreamOut(HMIDISTRM hMidiStrm, LPMIDIHDR lpMidiHdr,
 
     TRACE("(%p, %p, %u)!\n", hMidiStrm, lpMidiHdr, cbMidiHdr);
 
-    if (cbMidiHdr < sizeof(MIDIHDR) || !lpMidiHdr || !lpMidiHdr->lpData
+    if (cbMidiHdr < offsetof(MIDIHDR,dwOffset) || !lpMidiHdr || !lpMidiHdr->lpData
 	|| lpMidiHdr->dwBufferLength < lpMidiHdr->dwBytesRecorded)
 	return MMSYSERR_INVALPARAM;
     /* FIXME: Native additionaly checks if the MIDIEVENTs in lpData
diff --git a/include/mmsystem.h b/include/mmsystem.h
index dd9723af2cb..d147bf3b123 100644
--- a/include/mmsystem.h
+++ b/include/mmsystem.h
@@ -718,6 +718,7 @@ typedef struct midihdr_tag {
     DWORD	dwFlags;
     struct midihdr_tag *lpNext;
     DWORD_PTR	reserved;
+    /* Win 32 extended the structure with these 2 fields */
     DWORD	dwOffset;
     DWORD_PTR	dwReserved[8];
 } MIDIHDR, *LPMIDIHDR;