diff --git a/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.spec.ts b/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.spec.ts
index 1a22fca8192..a9308772347 100644
--- a/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.spec.ts
+++ b/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.spec.ts
@@ -40,6 +40,9 @@ class MockTeacherProjectService {
   isGroupNode() {}
   nodeHasWork() {}
   getMaxScoreForNode() {}
+  getNodeById() {
+    return { constraints: [] };
+  }
   getNode() {
     return new Node();
   }
@@ -121,7 +124,7 @@ function toggleLockNode() {
     });
     describe('when there is no teacherRemovalConstraint', () => {
       it('should add constraint', () => {
-        const getNodeSpy = spyOn(projectService, 'getNode').and.returnValue(node1);
+        const getNodeSpy = spyOn(projectService, 'getNodeById').and.returnValue(node1);
         expect(node1.constraints.length).toEqual(0);
         lockNodeButton.click();
         expect(getNodeSpy).toHaveBeenCalled();
@@ -141,7 +144,7 @@ function toggleLockNode() {
             Object({ name: 'teacherRemoval', params: Object({ periodId: periodId }) })
           ]
         });
-        const getNodeSpy = spyOn(projectService, 'getNode').and.returnValue(node1);
+        const getNodeSpy = spyOn(projectService, 'getNodeById').and.returnValue(node1);
         const dataService = TestBed.inject(TeacherDataService);
         const currentPeriodSpy = spyOn(dataService, 'getCurrentPeriod').and.returnValue({
           periodId: periodId
diff --git a/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts b/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts
index cdd994ac3b8..a4f7cd6ef73 100644
--- a/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts
+++ b/src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts
@@ -187,7 +187,7 @@ export class NavItemComponent implements OnInit {
   }
 
   isLocked(): boolean {
-    const constraints = this.projectService.getNode(this.nodeId).constraints;
+    const constraints = this.projectService.getNodeById(this.nodeId).constraints ?? [];
     return (
       (this.isShowingAllPeriods() && this.isLockedForAll(constraints)) ||
       (!this.isShowingAllPeriods() &&
@@ -218,7 +218,7 @@ export class NavItemComponent implements OnInit {
   }
 
   protected toggleLockNode(): void {
-    const node = this.projectService.getNode(this.nodeId);
+    const node = this.projectService.getNodeById(this.nodeId);
     const isLocked = this.isLocked();
     if (isLocked) {
       this.unlockNode(node);
@@ -270,7 +270,10 @@ export class NavItemComponent implements OnInit {
         }
       ]
     };
-    node.addConstraint(lockConstraint);
+    if (node.constraints == null) {
+      node.constraints = [];
+    }
+    node.constraints.push(lockConstraint);
   }
 
   private unlockNodeForAllPeriods(node: Node): void {
diff --git a/src/messages.xlf b/src/messages.xlf
index a3bc1b72bbd..99d87d209b6 100644
--- a/src/messages.xlf
+++ b/src/messages.xlf
@@ -14218,7 +14218,7 @@ Click &quot;Cancel&quot; to keep the invalid JSON open so you can fix it.</sourc
         <source>All Periods</source>
         <context-group purpose="location">
           <context context-type="sourcefile">src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts</context>
-          <context context-type="linenumber">371</context>
+          <context context-type="linenumber">374</context>
         </context-group>
         <context-group purpose="location">
           <context context-type="sourcefile">src/assets/wise5/classroomMonitor/classroomMonitorComponents/select-period/select-period.component.ts</context>
@@ -14233,7 +14233,7 @@ Click &quot;Cancel&quot; to keep the invalid JSON open so you can fix it.</sourc
         <source>Period: <x id="PH" equiv-text="this.currentPeriod.periodName"/></source>
         <context-group purpose="location">
           <context context-type="sourcefile">src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts</context>
-          <context context-type="linenumber">372</context>
+          <context context-type="linenumber">375</context>
         </context-group>
         <context-group purpose="location">
           <context context-type="sourcefile">src/assets/wise5/classroomMonitor/classroomMonitorComponents/select-period/select-period.component.ts</context>
@@ -14244,14 +14244,14 @@ Click &quot;Cancel&quot; to keep the invalid JSON open so you can fix it.</sourc
         <source>Unlock for <x id="PH" equiv-text="this.getPeriodLabel()"/></source>
         <context-group purpose="location">
           <context context-type="sourcefile">src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts</context>
-          <context context-type="linenumber">377</context>
+          <context context-type="linenumber">380</context>
         </context-group>
       </trans-unit>
       <trans-unit id="2330813372531032088" datatype="html">
         <source>Lock for <x id="PH" equiv-text="this.getPeriodLabel()"/></source>
         <context-group purpose="location">
           <context context-type="sourcefile">src/assets/wise5/classroomMonitor/classroomMonitorComponents/nodeProgress/nav-item/nav-item.component.ts</context>
-          <context context-type="linenumber">378</context>
+          <context context-type="linenumber">381</context>
         </context-group>
       </trans-unit>
       <trans-unit id="5411ee1bc7179f254f2db73cf448e995fca45f43" datatype="html">