diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index b8326124649e0e29a0a627d0c7886aa2500af534..3063c6a32fc2f3af8347c591ddead59afa0626c3 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -2549,23 +2549,46 @@ qemuCheckSCSIControllerIOThreads(const virDomainDef *domainDef,
 }
 
 
-char *
+/**
+ * qemuBuildControllerDevStr:
+ * @domainDef: domain definition
+ * @def: controller definition
+ * @qemuCaps: QEMU binary capabilities
+ * @devstr: device string
+ * @nusbcontroller: number of USB controllers
+ *
+ * Turn @def into a description of the controller that QEMU will understand,
+ * to be used either on the command line or through the monitor.
+ *
+ * The description will be returned in @devstr and can be NULL, eg. when
+ * passing in one of the built-in controllers. The returned string must be
+ * freed by the caller.
+ *
+ * The number pointed to by @nusbcontroller will be increased by one every
+ * time the description for a USB controller has been generated successfully.
+ *
+ * Returns: 0 on success, <0 on failure
+ */
+int
 qemuBuildControllerDevStr(const virDomainDef *domainDef,
                           virDomainControllerDefPtr def,
                           virQEMUCapsPtr qemuCaps,
+                          char **devstr,
                           int *nusbcontroller)
 {
     virBuffer buf = VIR_BUFFER_INITIALIZER;
     int model = def->model;
     const char *modelName = NULL;
 
+    *devstr = NULL;
+
     if (!qemuCheckCCWS390AddressSupport(domainDef, def->info, qemuCaps,
                                         "controller"))
-        return NULL;
+        return -1;
 
     if (def->type == VIR_DOMAIN_CONTROLLER_TYPE_SCSI) {
         if ((qemuDomainSetSCSIControllerModel(domainDef, qemuCaps, &model)) < 0)
-            return NULL;
+            return -1;
     }
 
     if (!(def->type == VIR_DOMAIN_CONTROLLER_TYPE_SCSI &&
@@ -2573,22 +2596,22 @@ qemuBuildControllerDevStr(const virDomainDef *domainDef,
         if (def->queues) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("'queues' is only supported by virtio-scsi controller"));
-            return NULL;
+            return -1;
         }
         if (def->cmd_per_lun) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("'cmd_per_lun' is only supported by virtio-scsi controller"));
-            return NULL;
+            return -1;
         }
         if (def->max_sectors) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("'max_sectors' is only supported by virtio-scsi controller"));
-            return NULL;
+            return -1;
         }
         if (def->ioeventfd) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("'ioeventfd' is only supported by virtio-scsi controller"));
-            return NULL;
+            return -1;
         }
     }
 
@@ -3003,11 +3026,12 @@ qemuBuildControllerDevStr(const virDomainDef *domainDef,
     if (virBufferCheckError(&buf) < 0)
         goto error;
 
-    return virBufferContentAndReset(&buf);
+    *devstr = virBufferContentAndReset(&buf);
+    return 0;
 
  error:
     virBufferFreeAndReset(&buf);
-    return NULL;
+    return -1;
 }
 
 
@@ -3102,12 +3126,15 @@ qemuBuildControllerDevCommandLine(virCommandPtr cmd,
                 continue;
             }
 
-            virCommandAddArg(cmd, "-device");
-            if (!(devstr = qemuBuildControllerDevStr(def, cont, qemuCaps,
-                                                     &usbcontroller)))
+            if (qemuBuildControllerDevStr(def, cont, qemuCaps,
+                                          &devstr, &usbcontroller) < 0)
                 return -1;
-            virCommandAddArg(cmd, devstr);
-            VIR_FREE(devstr);
+
+            if (devstr) {
+                virCommandAddArg(cmd, "-device");
+                virCommandAddArg(cmd, devstr);
+                VIR_FREE(devstr);
+            }
         }
     }
 
diff --git a/src/qemu/qemu_command.h b/src/qemu/qemu_command.h
index f5e3e5fbef5553a460342726ebbd6e3a7513cd5b..6fbfb3e5fa4a9577c055dc98885f5a4cdd0ae375 100644
--- a/src/qemu/qemu_command.h
+++ b/src/qemu/qemu_command.h
@@ -119,10 +119,11 @@ char *qemuBuildDriveDevStr(const virDomainDef *def,
                            virQEMUCapsPtr qemuCaps);
 
 /* Current, best practice */
-char *qemuBuildControllerDevStr(const virDomainDef *domainDef,
-                                virDomainControllerDefPtr def,
-                                virQEMUCapsPtr qemuCaps,
-                                int *nusbcontroller);
+int qemuBuildControllerDevStr(const virDomainDef *domainDef,
+                              virDomainControllerDefPtr def,
+                              virQEMUCapsPtr qemuCaps,
+                              char **devstr,
+                              int *nusbcontroller);
 
 int qemuBuildMemoryBackendStr(virJSONValuePtr *backendProps,
                               const char **backendType,
diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
index 090142db9b3a0a5ddf694ee951dd7e3aab76532d..4bc49720b552762c7f67e3c28148b884c8487e77 100644
--- a/src/qemu/qemu_hotplug.c
+++ b/src/qemu/qemu_hotplug.c
@@ -523,7 +523,10 @@ int qemuDomainAttachControllerDevice(virQEMUDriverPtr driver,
     if (qemuAssignDeviceControllerAlias(vm->def, priv->qemuCaps, controller) < 0)
         goto cleanup;
 
-    if (!(devstr = qemuBuildControllerDevStr(vm->def, controller, priv->qemuCaps, NULL)))
+    if (qemuBuildControllerDevStr(vm->def, controller, priv->qemuCaps, &devstr, NULL) < 0)
+        goto cleanup;
+
+    if (!devstr)
         goto cleanup;
 
     if (VIR_REALLOC_N(vm->def->controllers, vm->def->ncontrollers+1) < 0)