diff --git a/driver/linux b/driver/linux
index afea42a..3eeb1de 160000
--- a/driver/linux
+++ b/driver/linux
@@ -1 +1 @@
-Subproject commit afea42a6cd9592170fb6fff362e998b3706a7512
+Subproject commit 3eeb1de32dc117cf58114ca7775b94a2197b0b2f
diff --git a/inc/hf/api.h b/inc/hf/api.h
index 4060075..43497bf 100644
--- a/inc/hf/api.h
+++ b/inc/hf/api.h
@@ -25,7 +25,8 @@
 void api_init(struct mpool *ppool);
 spci_vm_id_t api_vm_get_id(const struct vcpu *current);
 spci_vm_count_t api_vm_get_count(void);
-int64_t api_vcpu_get_count(spci_vm_id_t vm_id, const struct vcpu *current);
+spci_vcpu_count_t api_vcpu_get_count(spci_vm_id_t vm_id,
+				     const struct vcpu *current);
 void api_regs_state_saved(struct vcpu *vcpu);
 struct hf_vcpu_run_return api_vcpu_run(spci_vm_id_t vm_id,
 				       spci_vcpu_index_t vcpu_idx,
diff --git a/inc/hf/vm.h b/inc/hf/vm.h
index bd8a905..c86da21 100644
--- a/inc/hf/vm.h
+++ b/inc/hf/vm.h
@@ -78,7 +78,7 @@
 	spci_vm_id_t id;
 	/** See api.c for the partial ordering on locks. */
 	struct spinlock lock;
-	uint32_t vcpu_count;
+	spci_vcpu_count_t vcpu_count;
 	struct vcpu vcpus[MAX_CPUS];
 	struct mm_ptable ptable;
 	struct mailbox mailbox;
@@ -97,7 +97,8 @@
 	struct vm *vm;
 };
 
-bool vm_init(uint32_t vcpu_count, struct mpool *ppool, struct vm **new_vm);
+bool vm_init(spci_vcpu_count_t vcpu_count, struct mpool *ppool,
+	     struct vm **new_vm);
 spci_vm_count_t vm_get_count(void);
 struct vm *vm_find(spci_vm_id_t id);
 struct vm_locked vm_lock(struct vm *vm);
diff --git a/inc/vmapi/hf/call.h b/inc/vmapi/hf/call.h
index 3ba985c..6e68a50 100644
--- a/inc/vmapi/hf/call.h
+++ b/inc/vmapi/hf/call.h
@@ -64,7 +64,7 @@
 /**
  * Returns the number of VCPUs configured in the given secondary VM.
  */
-static inline int64_t hf_vcpu_get_count(spci_vm_id_t vm_id)
+static inline spci_vcpu_count_t hf_vcpu_get_count(spci_vm_id_t vm_id)
 {
 	return hf_call(HF_VCPU_GET_COUNT, vm_id, 0, 0);
 }
diff --git a/inc/vmapi/hf/spci.h b/inc/vmapi/hf/spci.h
index 25e8e83..e55ea97 100644
--- a/inc/vmapi/hf/spci.h
+++ b/inc/vmapi/hf/spci.h
@@ -68,8 +68,16 @@
  * different name to make the different semantics clear.
  */
 typedef spci_vm_id_t spci_vm_count_t;
+
+/** The index of a vCPU within a particular VM. */
 typedef uint16_t spci_vcpu_index_t;
 
+/**
+ * A count of vCPUs. This has the same range as the vCPU indices but we give it
+ * a different name to make the different semantics clear.
+ */
+typedef spci_vcpu_index_t spci_vcpu_count_t;
+
 /** SPCI common message header. */
 struct spci_message {
 	/*
diff --git a/src/api.c b/src/api.c
index 5506a04..bb954a6 100644
--- a/src/api.c
+++ b/src/api.c
@@ -224,20 +224,22 @@
 }
 
 /**
- * Returns the number of vcpus configured in the given VM.
+ * Returns the number of vCPUs configured in the given VM, or 0 if there is no
+ * such VM or the caller is not the primary VM.
  */
-int64_t api_vcpu_get_count(spci_vm_id_t vm_id, const struct vcpu *current)
+spci_vcpu_count_t api_vcpu_get_count(spci_vm_id_t vm_id,
+				     const struct vcpu *current)
 {
 	struct vm *vm;
 
 	/* Only the primary VM needs to know about vcpus for scheduling. */
 	if (current->vm->id != HF_PRIMARY_VM_ID) {
-		return -1;
+		return 0;
 	}
 
 	vm = vm_find(vm_id);
 	if (vm == NULL) {
-		return -1;
+		return 0;
 	}
 
 	return vm->vcpu_count;
diff --git a/src/vm.c b/src/vm.c
index 0119fd8..f1b79e6 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -27,7 +27,8 @@
 static struct vm vms[MAX_VMS];
 static spci_vm_count_t vm_count;
 
-bool vm_init(uint32_t vcpu_count, struct mpool *ppool, struct vm **new_vm)
+bool vm_init(spci_vcpu_count_t vcpu_count, struct mpool *ppool,
+	     struct vm **new_vm)
 {
 	uint32_t i;
 	struct vm *vm;
diff --git a/test/vmapi/primary_only/primary_only.c b/test/vmapi/primary_only/primary_only.c
index 3125b20..c57561d 100644
--- a/test/vmapi/primary_only/primary_only.c
+++ b/test/vmapi/primary_only/primary_only.c
@@ -54,7 +54,7 @@
  */
 TEST(hf_vcpu_get_count, no_secondary_vms)
 {
-	EXPECT_EQ(hf_vcpu_get_count(1), -1);
+	EXPECT_EQ(hf_vcpu_get_count(1), 0);
 }
 
 /**
@@ -63,7 +63,7 @@
  */
 TEST(hf_vcpu_get_count, large_invalid_vm_index)
 {
-	EXPECT_EQ(hf_vcpu_get_count(0xffff), -1);
+	EXPECT_EQ(hf_vcpu_get_count(0xffff), 0);
 }
 
 /**
diff --git a/test/vmapi/primary_with_secondaries/no_services.c b/test/vmapi/primary_with_secondaries/no_services.c
index e2c52f1..7b171e8 100644
--- a/test/vmapi/primary_with_secondaries/no_services.c
+++ b/test/vmapi/primary_with_secondaries/no_services.c
@@ -64,7 +64,7 @@
  */
 TEST(hf_vcpu_get_count, large_invalid_vm_index)
 {
-	EXPECT_EQ(hf_vcpu_get_count(0xffff), -1);
+	EXPECT_EQ(hf_vcpu_get_count(0xffff), 0);
 }
 
 /**