diff --git a/src/dll/fsctl.c b/src/dll/fsctl.c
index cab35fff..9237655a 100644
--- a/src/dll/fsctl.c
+++ b/src/dll/fsctl.c
@@ -30,6 +30,7 @@ FSP_API NTSTATUS FspFsctlCreateVolume(PWSTR DevicePath, PSECURITY_DESCRIPTOR Sec
 
     GlobalDevicePath(DevicePathBuf, sizeof DevicePathBuf, DevicePath);
 
+    SecurityDescriptorSize = 0;
     if (!MakeSelfRelativeSD(SecurityDescriptor, 0, &SecurityDescriptorSize))
     {
         SecurityDescriptorBuf = malloc(SecurityDescriptorSize);