diff --git a/provider/client/WSUtil.cpp b/provider/client/WSUtil.cpp
index 2bfa558ec..64d99fa9f 100644
--- a/provider/client/WSUtil.cpp
+++ b/provider/client/WSUtil.cpp
@@ -2077,7 +2077,14 @@ static HRESULT CopyMAPISourceKeyToSoapSourceKey(const SBinary *lpsMAPISourceKey,
 {
 	if (lpsMAPISourceKey == nullptr || lpsSoapSourceKey == nullptr)
 		return MAPI_E_INVALID_PARAMETER;
-	lpsSoapSourceKey->__ptr  = soap_new_unsignedByte(nullptr, lpsSoapSourceKey->__size);
+	if (lpsMAPISourceKey->lpb == nullptr) {
+		lpsSoapSourceKey->__ptr = nullptr;
+		lpsSoapSourceKey->__size = 0;
+		return hrSuccess;
+	}
+	lpsSoapSourceKey->__ptr = soap_new_unsignedByte(nullptr, lpsMAPISourceKey->cb);
+	if (lpsSoapSourceKey->__ptr == nullptr)
+		throw std::bad_alloc();
 	lpsSoapSourceKey->__size = lpsMAPISourceKey->cb;
 	memcpy(lpsSoapSourceKey->__ptr, lpsMAPISourceKey->lpb, lpsSoapSourceKey->__size);
 	return hrSuccess;