diff --git a/components/auto-complete/nz-autocomplete-trigger.directive.ts b/components/auto-complete/nz-autocomplete-trigger.directive.ts
index de401d5935f..9d160be0319 100644
--- a/components/auto-complete/nz-autocomplete-trigger.directive.ts
+++ b/components/auto-complete/nz-autocomplete-trigger.directive.ts
@@ -315,8 +315,9 @@ export class NzAutocompleteTriggerDirective implements ControlValueAccessor, OnD
   }
 
   private resetActiveItem(): void {
-    if (this.nzAutocomplete.activeItem && this.nzAutocomplete.getOptionIndex(this.nzAutocomplete.activeItem)) {
-      this.nzAutocomplete.setActiveItem(this.nzAutocomplete.getOptionIndex(this.nzAutocomplete.activeItem));
+    const index = this.nzAutocomplete.getOptionIndex(this.nzAutocomplete.activeItem);
+    if (this.nzAutocomplete.activeItem && index !== -1) {
+      this.nzAutocomplete.setActiveItem(index);
     } else {
       this.nzAutocomplete.setActiveItem(this.nzAutocomplete.nzDefaultActiveFirstOption ? 0 : -1);
     }
diff --git a/components/auto-complete/nz-autocomplete.component.ts b/components/auto-complete/nz-autocomplete.component.ts
index 083ee7300e1..b2a9617a170 100644
--- a/components/auto-complete/nz-autocomplete.component.ts
+++ b/components/auto-complete/nz-autocomplete.component.ts
@@ -147,7 +147,7 @@ export class NzAutocompleteComponent implements AfterViewInit, OnDestroy {
 
   getOptionIndex(option?: NzAutocompleteOptionComponent): number {
     return this.options.reduce((result: number, current: NzAutocompleteOptionComponent, index: number) => {
-      return result === undefined ? (option === current ? index : -1) : result;
+      return result === -1 ? (option === current ? index : -1) : result;
     }, -1)!;
   }
 
diff --git a/components/form/nz-form-control.component.ts b/components/form/nz-form-control.component.ts
index fa1c183659e..209e46de3c4 100644
--- a/components/form/nz-form-control.component.ts
+++ b/components/form/nz-form-control.component.ts
@@ -40,7 +40,7 @@ import { NzFormItemComponent } from './nz-form-item.component';
 export class NzFormControlComponent extends NzColDirective implements OnDestroy, OnInit, AfterContentInit, AfterViewInit, OnDestroy {
   private _hasFeedback = false;
   validateChanges: Subscription | null;
-  validateString: string;
+  validateString: string | null;
   controlClassMap: NgClassType = {};
   iconType: string;
   validateControl: FormControl | null;
@@ -60,11 +60,11 @@ export class NzFormControlComponent extends NzColDirective implements OnDestroy,
   set nzValidateStatus(value: string | FormControl | FormControlName) {
     if (value instanceof FormControl) {
       this.validateControl = value;
-      this.validateString = '';
+      this.validateString = null;
       this.watchControl();
     } else if (value instanceof FormControlName) {
       this.validateControl = value.control;
-      this.validateString = '';
+      this.validateString = null;
       this.watchControl();
     } else {
       this.validateString = value;