diff --git a/backends/tpm.c b/backends/tpm.c
index 99d2b2b..c2be17b 100644
--- a/backends/tpm.c
+++ b/backends/tpm.c
@@ -48,14 +48,16 @@ const char *tpm_backend_get_desc(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
-    return k->ops->desc();
+    return k->ops->desc ? k->ops->desc() : "";
 }
 
 void tpm_backend_destroy(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
-    k->ops->destroy(s);
+    if (k->ops->destroy) {
+        k->ops->destroy(s);
+    }
 
     object_unref(OBJECT(s));
 }
@@ -68,7 +70,7 @@ int tpm_backend_init(TPMBackend *s, TPMState *state,
     s->tpm_state = state;
     s->recv_data_callback = datacb;
 
-    return k->ops->init(s);
+    return k->ops->init ? k->ops->init(s) : 0;
 }
 
 int tpm_backend_startup_tpm(TPMBackend *s)
@@ -82,13 +84,15 @@ int tpm_backend_startup_tpm(TPMBackend *s)
                                        NULL);
     g_thread_pool_push(s->thread_pool, (gpointer)TPM_BACKEND_CMD_INIT, NULL);
 
-    return k->ops->startup_tpm(s);
+    return k->ops->startup_tpm ? k->ops->startup_tpm(s) : 0;
 }
 
 bool tpm_backend_had_startup_error(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
+    assert(k->ops->had_startup_error);
+
     return k->ops->had_startup_error(s);
 }
 
@@ -96,6 +100,8 @@ size_t tpm_backend_realloc_buffer(TPMBackend *s, TPMSizedBuffer *sb)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
+    assert(k->ops->realloc_buffer);
+
     return k->ops->realloc_buffer(sb);
 }
 
@@ -109,7 +115,9 @@ void tpm_backend_reset(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
-    k->ops->reset(s);
+    if (k->ops->reset) {
+        k->ops->reset(s);
+    }
 
     tpm_backend_thread_end(s);
 }
@@ -118,6 +126,8 @@ void tpm_backend_cancel_cmd(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
+    g_assert(k->ops->cancel_cmd);
+
     k->ops->cancel_cmd(s);
 }
 
@@ -125,20 +135,24 @@ bool tpm_backend_get_tpm_established_flag(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
-    return k->ops->get_tpm_established_flag(s);
+    return k->ops->get_tpm_established_flag ?
+           k->ops->get_tpm_established_flag(s) : false;
 }
 
 int tpm_backend_reset_tpm_established_flag(TPMBackend *s, uint8_t locty)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
-    return k->ops->reset_tpm_established_flag(s, locty);
+    return k->ops->reset_tpm_established_flag ?
+           k->ops->reset_tpm_established_flag(s, locty) : 0;
 }
 
 TPMVersion tpm_backend_get_tpm_version(TPMBackend *s)
 {
     TPMBackendClass *k = TPM_BACKEND_GET_CLASS(s);
 
+    assert(k->ops->get_tpm_version);
+
     return k->ops->get_tpm_version(s);
 }
 
diff --git a/hw/tpm/tpm_passthrough.c b/hw/tpm/tpm_passthrough.c
index 65831b5..0f543bd 100644
--- a/hw/tpm/tpm_passthrough.c
+++ b/hw/tpm/tpm_passthrough.c
@@ -227,15 +227,6 @@ static void tpm_passthrough_handle_request(TPMBackend *tb, TPMBackendCmd cmd)
     }
 }
 
-/*
- * Start the TPM (thread). If it had been started before, then terminate
- * and start it again.
- */
-static int tpm_passthrough_startup_tpm(TPMBackend *tb)
-{
-    return 0;
-}
-
 static void tpm_passthrough_reset(TPMBackend *tb)
 {
     TPMPassthruState *tpm_pt = TPM_PASSTHROUGH(tb);
@@ -247,11 +238,6 @@ static void tpm_passthrough_reset(TPMBackend *tb)
     tpm_pt->had_startup_error = false;
 }
 
-static int tpm_passthrough_init(TPMBackend *tb)
-{
-    return 0;
-}
-
 static bool tpm_passthrough_get_tpm_established_flag(TPMBackend *tb)
 {
     return false;
@@ -467,8 +453,6 @@ static const TPMDriverOps tpm_passthrough_driver = {
     .desc                     = tpm_passthrough_create_desc,
     .create                   = tpm_passthrough_create,
     .destroy                  = tpm_passthrough_destroy,
-    .init                     = tpm_passthrough_init,
-    .startup_tpm              = tpm_passthrough_startup_tpm,
     .realloc_buffer           = tpm_passthrough_realloc_buffer,
     .reset                    = tpm_passthrough_reset,
     .had_startup_error        = tpm_passthrough_get_startup_error,
