diff --git a/src/compiler/builder.ts b/src/compiler/builder.ts
index e2d59975d04..f4a61267144 100644
--- a/src/compiler/builder.ts
+++ b/src/compiler/builder.ts
@@ -65,7 +65,8 @@ namespace ts {
         }
         state.changedFilesSet = createMap<true>();
         const useOldState = BuilderState.canReuseOldState(state.referencedMap, oldState);
-        const canCopySemanticDiagnostics = useOldState && oldState!.semanticDiagnosticsPerFile && !!state.semanticDiagnosticsPerFile;
+        const canCopySemanticDiagnostics = useOldState && oldState!.semanticDiagnosticsPerFile && !!state.semanticDiagnosticsPerFile &&
+            !compilerOptionsAffectSemanticDiagnostics(compilerOptions, oldState!.program.getCompilerOptions());
         if (useOldState) {
             // Verify the sanity of old state
             if (!oldState!.currentChangedFilePath) {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 56ea3cace0c..dcc4f968f26 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -6967,6 +6967,49 @@ namespace ts {
         return compilerOptions[flag] === undefined ? !!compilerOptions.strict : !!compilerOptions[flag];
     }
 
+    export function compilerOptionsAffectSemanticDiagnostics(newOptions: CompilerOptions, oldOptions: CompilerOptions) {
+        if (oldOptions === newOptions) {
+            return false;
+        }
+
+        return changedCompileOptionValueOf(newOptions, oldOptions, [
+            "noImplicitReturns",
+            "strict",
+            "suppressExcessPropertyErrors",
+            "suppressImplicitAnyIndexErrors",
+            "noFallthroughCasesInSwitch",
+            "noStrictGenericChecks",
+            "noUnusedLocals",
+            "noUnusedParameters",
+            "noImplicitUseStrict"
+        ]) || changedStrictOptionValueOf(newOptions, oldOptions, [
+            "noImplicitAny",
+            "noImplicitThis",
+            "strictNullChecks",
+            "strictFunctionTypes",
+            "strictPropertyInitialization",
+            "alwaysStrict"
+        ]);
+    }
+
+    function changedStrictOptionValueOf(newOptions: CompilerOptions, oldOptions: CompilerOptions, flags: StrictOptionName[]) {
+        for (const flag of flags) {
+            if (getStrictOptionValue(newOptions, flag) !== getStrictOptionValue(oldOptions, flag)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    function changedCompileOptionValueOf(newOptions: CompilerOptions, oldOptions: CompilerOptions, optionKeys: (keyof CompilerOptions)[]) {
+        for (const optionKey of optionKeys) {
+            if (newOptions[optionKey] !== oldOptions[optionKey]) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     export function hasZeroOrOneAsteriskCharacter(str: string): boolean {
         let seenAsterisk = false;
         for (let i = 0; i < str.length; i++) {
diff --git a/src/testRunner/unittests/tscWatchMode.ts b/src/testRunner/unittests/tscWatchMode.ts
index 1456a621019..d91559c72fc 100644
--- a/src/testRunner/unittests/tscWatchMode.ts
+++ b/src/testRunner/unittests/tscWatchMode.ts
@@ -1312,6 +1312,49 @@ export class B
             // File a need not be rewritten
             assert.equal(host.getModifiedTime(`${currentDirectory}/a.js`), modifiedTimeOfAJs);
         });
+
+        it("updates errors when strictNullChecks changes", () => {
+            const currentDirectory = "/user/username/projects/myproject";
+            const aFile: File = {
+                path: `${currentDirectory}/a.ts`,
+                content: `declare function foo(): null | { hello: any };
+foo().hello`
+            };
+            const compilerOptions: CompilerOptions = {
+            };
+            const config: File = {
+                path: `${currentDirectory}/tsconfig.json`,
+                content: JSON.stringify({ compilerOptions })
+            };
+            const files = [aFile, config, libFile];
+            const host = createWatchedSystem(files, { currentDirectory });
+            const watch = createWatchOfConfigFile("tsconfig.json", host);
+            checkProgramActualFiles(watch(), [aFile.path, libFile.path]);
+            checkOutputErrorsInitial(host, emptyArray);
+            const modifiedTimeOfAJs = host.getModifiedTime(`${currentDirectory}/a.js`);
+            compilerOptions.strictNullChecks = true;
+            host.writeFile(config.path, JSON.stringify({ compilerOptions }));
+            host.runQueuedTimeoutCallbacks();
+            const expectedStrictNullErrors = [
+                getDiagnosticOfFileFromProgram(watch(), aFile.path, aFile.content.lastIndexOf("foo()"), 5, Diagnostics.Object_is_possibly_null)
+            ];
+            checkOutputErrorsIncremental(host, expectedStrictNullErrors);
+            // File a need not be rewritten
+            assert.equal(host.getModifiedTime(`${currentDirectory}/a.js`), modifiedTimeOfAJs);
+            compilerOptions.strict = true;
+            delete (compilerOptions.strictNullChecks);
+            host.writeFile(config.path, JSON.stringify({ compilerOptions }));
+            host.runQueuedTimeoutCallbacks();
+            checkOutputErrorsIncremental(host, expectedStrictNullErrors);
+            // File a need not be rewritten
+            assert.equal(host.getModifiedTime(`${currentDirectory}/a.js`), modifiedTimeOfAJs);
+            delete (compilerOptions.strict);
+            host.writeFile(config.path, JSON.stringify({ compilerOptions }));
+            host.runQueuedTimeoutCallbacks();
+            checkOutputErrorsIncremental(host, emptyArray);
+            // File a need not be rewritten
+            assert.equal(host.getModifiedTime(`${currentDirectory}/a.js`), modifiedTimeOfAJs);
+        });
     });
 
     describe("tsc-watch emit with outFile or out setting", () => {