diff --git a/windows/syscall_windows.go b/windows/syscall_windows.go
index 74b72a0b..1d1170ab 100644
--- a/windows/syscall_windows.go
+++ b/windows/syscall_windows.go
@@ -288,8 +288,8 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	SetVolumeLabel(rootPathName *uint16, volumeName *uint16) (err error) = SetVolumeLabelW
 //sys	SetVolumeMountPoint(volumeMountPoint *uint16, volumeName *uint16) (err error) = SetVolumeMountPointW
 //sys	MessageBox(hwnd Handle, text *uint16, caption *uint16, boxtype uint32) (ret int32, err error) [failretval==0] = user32.MessageBoxW
-//sys	clsidFromString(lpsz *uint16, pclsid *GUID) (err error) [failretval!=0] = ole32.CLSIDFromString
-//sys	stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int) (chars int) = ole32.StringFromGUID2
+//sys	clsidFromString(lpsz *uint16, pclsid *GUID) (ret error) = ole32.CLSIDFromString
+//sys	stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int32) (chars int32) = ole32.StringFromGUID2
 //sys	coCreateGuid(pguid *GUID) (ret error) = ole32.CoCreateGuid
 
 // syscall interface implementation for other packages
@@ -1276,7 +1276,7 @@ func GenerateGUID() (GUID, error) {
 // in the form of "{XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}".
 func (guid GUID) String() string {
 	var str [100]uint16
-	chars := stringFromGUID2(&guid, &str[0], len(str))
+	chars := stringFromGUID2(&guid, &str[0], int32(len(str)))
 	if chars <= 1 {
 		return ""
 	}
diff --git a/windows/syscall_windows_test.go b/windows/syscall_windows_test.go
index b157625f..9f78e2d9 100644
--- a/windows/syscall_windows_test.go
+++ b/windows/syscall_windows_test.go
@@ -191,4 +191,8 @@ func TestGUID(t *testing.T) {
 	if guid2 != guid {
 		t.Fatalf("Did not parse string back to original GUID = %q; want %q", guid2, guid)
 	}
+	_, err = windows.GUIDFromString("not-a-real-guid")
+	if err != syscall.Errno(windows.CO_E_CLASSSTRING) {
+		t.Fatalf("Bad GUID string error = %v; want CO_E_CLASSSTRING", err)
+	}
 }
diff --git a/windows/zsyscall_windows.go b/windows/zsyscall_windows.go
index f37d7d42..6c35b90c 100644
--- a/windows/zsyscall_windows.go
+++ b/windows/zsyscall_windows.go
@@ -2436,21 +2436,17 @@ func MessageBox(hwnd Handle, text *uint16, caption *uint16, boxtype uint32) (ret
 	return
 }
 
-func clsidFromString(lpsz *uint16, pclsid *GUID) (err error) {
-	r1, _, e1 := syscall.Syscall(procCLSIDFromString.Addr(), 2, uintptr(unsafe.Pointer(lpsz)), uintptr(unsafe.Pointer(pclsid)), 0)
-	if r1 != 0 {
-		if e1 != 0 {
-			err = errnoErr(e1)
-		} else {
-			err = syscall.EINVAL
-		}
+func clsidFromString(lpsz *uint16, pclsid *GUID) (ret error) {
+	r0, _, _ := syscall.Syscall(procCLSIDFromString.Addr(), 2, uintptr(unsafe.Pointer(lpsz)), uintptr(unsafe.Pointer(pclsid)), 0)
+	if r0 != 0 {
+		ret = syscall.Errno(r0)
 	}
 	return
 }
 
-func stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int) (chars int) {
+func stringFromGUID2(rguid *GUID, lpsz *uint16, cchMax int32) (chars int32) {
 	r0, _, _ := syscall.Syscall(procStringFromGUID2.Addr(), 3, uintptr(unsafe.Pointer(rguid)), uintptr(unsafe.Pointer(lpsz)), uintptr(cchMax))
-	chars = int(r0)
+	chars = int32(r0)
 	return
 }