diff --git a/extension/rikaicontent.ts b/extension/rikaicontent.ts
index b09922d0f..ca670c801 100644
--- a/extension/rikaicontent.ts
+++ b/extension/rikaicontent.ts
@@ -1094,7 +1094,14 @@ class RcxContent {
         fake.scrollLeft = eventTarget.scrollLeft;
       }
       // Calculate range and friends here after we've made our fake textarea/input divs.
-      range = document.caretRangeFromPoint(ev.clientX, ev.clientY);
+      range = document.caretRangeFromPoint(
+        ev.clientX,
+        ev.clientY
+      ) as Range | null;
+      // If we don't have a valid range, don't do any more work
+      if (range === null) {
+        return;
+      }
       const startNode = range.startContainer;
       ro = range.startOffset;
 
diff --git a/extension/test/rikaicontent_test.ts b/extension/test/rikaicontent_test.ts
index e0fd45250..e2a29c5a3 100644
--- a/extension/test/rikaicontent_test.ts
+++ b/extension/test/rikaicontent_test.ts
@@ -1,61 +1,80 @@
+import { Config } from '../configuration';
 import { TestOnlyRcxContent } from '../rikaicontent';
 import { expect, use } from '@esm-bundle/chai';
 import chrome from 'sinon-chrome';
+import simulant from 'simulant';
 import sinon from 'sinon';
 import sinonChai from 'sinon-chai';
 
 use(sinonChai);
 
-describe('RcxContent.show', () => {
+let rcxContent = new TestOnlyRcxContent();
+
+describe('RcxContent', () => {
   beforeEach(() => {
     chrome.reset();
+    rcxContent = new TestOnlyRcxContent();
+    // Default enable rcxContent since no tests care about that now.
+    rcxContent.enableTab({ showOnKey: '' } as Config);
   });
+  describe('.show', () => {
+    describe('when given Japanese word interrupted with text wrapped by `display: none`', () => {
+      it('sends "xsearch" message with invisible text omitted', () => {
+        const span = insertHtmlIntoDomAndReturnFirstTextNode(
+          '<span>試<span style="display:none">test</span>す</span>'
+        );
+
+        executeShowForGivenNode(rcxContent, span);
 
-  describe('when given Japanese word interrupted with text wrapped by `display: none`', () => {
-    it('sends "xsearch" message with invisible text omitted', () => {
-      const rcxContent = new TestOnlyRcxContent();
-      const span = insertHtmlIntoDomAndReturnFirstTextNode(
-        '<span>試<span style="display:none">test</span>す</span>'
-      );
+        expect(chrome.runtime.sendMessage).to.have.been.calledWith(
+          sinon.match({ type: 'xsearch', text: '試す' }),
+          sinon.match.any
+        );
+      });
+    });
 
-      executeShowForGivenNode(rcxContent, span);
+    describe('when given Japanese word interrupted with text wrapped by `visibility: hidden`', () => {
+      it('sends "xsearch" message with invisible text omitted', () => {
+        const span = insertHtmlIntoDomAndReturnFirstTextNode(
+          '<span>試<span style="visibility: hidden">test</span>す</span>'
+        );
 
-      expect(chrome.runtime.sendMessage).to.have.been.calledWith(
-        sinon.match({ type: 'xsearch', text: '試す' }),
-        sinon.match.any
-      );
+        executeShowForGivenNode(rcxContent, span);
+
+        expect(chrome.runtime.sendMessage).to.have.been.calledWith(
+          sinon.match({ type: 'xsearch', text: '試す' }),
+          sinon.match.any
+        );
+      });
     });
-  });
 
-  describe('when given Japanese word interrupted with text wrapped by `visibility: hidden`', () => {
-    it('sends "xsearch" message with invisible text omitted', () => {
-      const rcxContent = new TestOnlyRcxContent();
-      const span = insertHtmlIntoDomAndReturnFirstTextNode(
-        '<span>試<span style="visibility: hidden">test</span>す</span>'
-      );
+    describe('when given Japanese word is interrupted with text wrapped by visible span', () => {
+      it('sends "xsearch" message with all text included', () => {
+        const rcxContent = new TestOnlyRcxContent();
+        const span = insertHtmlIntoDomAndReturnFirstTextNode(
+          '<span>試<span>test</span>す</span>'
+        );
 
-      executeShowForGivenNode(rcxContent, span);
+        executeShowForGivenNode(rcxContent, span);
 
-      expect(chrome.runtime.sendMessage).to.have.been.calledWith(
-        sinon.match({ type: 'xsearch', text: '試す' }),
-        sinon.match.any
-      );
+        expect(chrome.runtime.sendMessage).to.have.been.calledWith(
+          sinon.match({ type: 'xsearch', text: '試testす' }),
+          sinon.match.any
+        );
+      });
     });
   });
 
-  describe('when given Japanese word is interrupted with text wrapped by visible span', () => {
-    it('sends "xsearch" message with all text included', () => {
-      const rcxContent = new TestOnlyRcxContent();
-      const span = insertHtmlIntoDomAndReturnFirstTextNode(
-        '<span>試<span>test</span>す</span>'
-      );
+  describe('mousemove', () => {
+    it('handled without logging errors if `caretRangeFromPoint` returns null', () => {
+      sinon
+        .stub(document, 'caretRangeFromPoint')
+        .returns(null as unknown as Range);
+      sinon.spy(console, 'log');
 
-      executeShowForGivenNode(rcxContent, span);
+      simulant.fire(document, 'mousemove');
 
-      expect(chrome.runtime.sendMessage).to.have.been.calledWith(
-        sinon.match({ type: 'xsearch', text: '試testす' }),
-        sinon.match.any
-      );
+      expect(console.log).to.not.have.been.called;
     });
   });
 });
diff --git a/package-lock.json b/package-lock.json
index f6fb48c40..eb3ac045b 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1785,6 +1785,12 @@
         "@types/node": "*"
       }
     },
+    "@types/simulant": {
+      "version": "0.2.0",
+      "resolved": "https://registry.npmjs.org/@types/simulant/-/simulant-0.2.0.tgz",
+      "integrity": "sha512-pQcnO5/JMR9KEnQGuYkDNQ9IDFAp0nrCfCjxqZ03WY2QDcuMPR6w0VpL6MO5VQEn93YkNCW9nTuRl/q0+iasVg==",
+      "dev": true
+    },
     "@types/sinon": {
       "version": "10.0.2",
       "resolved": "https://registry.npmjs.org/@types/sinon/-/sinon-10.0.2.tgz",
@@ -12329,6 +12335,12 @@
         }
       }
     },
+    "simulant": {
+      "version": "0.2.2",
+      "resolved": "https://registry.npmjs.org/simulant/-/simulant-0.2.2.tgz",
+      "integrity": "sha1-8bzlJxK2p6DaON392n6DsgsdoB4=",
+      "dev": true
+    },
     "sinon": {
       "version": "7.5.0",
       "resolved": "https://registry.npmjs.org/sinon/-/sinon-7.5.0.tgz",
diff --git a/package.json b/package.json
index 7a972b357..6c4715f50 100644
--- a/package.json
+++ b/package.json
@@ -58,6 +58,7 @@
     "@types/chrome": "0.0.147",
     "@types/mocha": "^8.2.2",
     "@types/node": "^16.3.3",
+    "@types/simulant": "^0.2.0",
     "@types/sinon-chai": "^3.2.5",
     "@types/sinon-chrome": "^2.2.10",
     "@web/test-runner": "^0.13.13",
@@ -78,6 +79,7 @@
     "prettier-plugin-jsdoc": "^0.3.23",
     "semantic-release": "^17.4.4",
     "semantic-release-chrome": "^1.1.3",
+    "simulant": "^0.2.2",
     "sinon": "^7.5.0",
     "sinon-chai": "^3.7.0",
     "sinon-chrome": "^3.0.1",