diff --git a/module/spring-boot-mustache/build.gradle b/module/spring-boot-mustache/build.gradle
index 62b0ecb8af9..d3c79b70ae5 100644
--- a/module/spring-boot-mustache/build.gradle
+++ b/module/spring-boot-mustache/build.gradle
@@ -46,5 +46,11 @@ dependencies {
 	testImplementation(testFixtures(project(":core:spring-boot-autoconfigure")))
 	testImplementation("io.projectreactor:reactor-test")
 
+	testCompileOnly("com.google.code.findbugs:jsr305")
+
 	testRuntimeOnly("ch.qos.logback:logback-classic")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}
diff --git a/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/autoconfigure/MustacheAutoConfigurationServletIntegrationTests.java b/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/autoconfigure/MustacheAutoConfigurationServletIntegrationTests.java
index e33e4410d9f..46936b5419c 100644
--- a/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/autoconfigure/MustacheAutoConfigurationServletIntegrationTests.java
+++ b/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/autoconfigure/MustacheAutoConfigurationServletIntegrationTests.java
@@ -34,6 +34,7 @@ import org.springframework.boot.resttestclient.TestRestTemplate;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
 import org.springframework.boot.tomcat.autoconfigure.servlet.TomcatServletWebServerAutoConfiguration;
+import org.springframework.boot.web.server.WebServer;
 import org.springframework.boot.web.server.servlet.context.ServletWebServerApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -63,7 +64,9 @@ class MustacheAutoConfigurationServletIntegrationTests {
 
 	@BeforeEach
 	void init() {
-		this.port = this.context.getWebServer().getPort();
+		WebServer webServer = this.context.getWebServer();
+		assertThat(webServer).isNotNull();
+		this.port = webServer.getPort();
 	}
 
 	@Test
diff --git a/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/reactive/view/MustacheViewResolverTests.java b/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/reactive/view/MustacheViewResolverTests.java
index 6e177a633fa..86034bc5c23 100644
--- a/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/reactive/view/MustacheViewResolverTests.java
+++ b/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/reactive/view/MustacheViewResolverTests.java
@@ -17,6 +17,7 @@
 package org.springframework.boot.mustache.reactive.view;
 
 import java.time.Duration;
+import java.util.Locale;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -46,13 +47,13 @@ class MustacheViewResolverTests {
 
 	@Test
 	void resolveNonExistent() {
-		assertThat(this.resolver.resolveViewName("bar", null).block(Duration.ofSeconds(30))).isNull();
+		assertThat(this.resolver.resolveViewName("bar", Locale.ROOT).block(Duration.ofSeconds(30))).isNull();
 	}
 
 	@Test
 	@WithResource(name = "template.html", content = "Hello {{World}}")
 	void resolveExisting() {
-		assertThat(this.resolver.resolveViewName("template", null).block(Duration.ofSeconds(30))).isNotNull();
+		assertThat(this.resolver.resolveViewName("template", Locale.ROOT).block(Duration.ofSeconds(30))).isNotNull();
 	}
 
 }
diff --git a/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/servlet/view/MustacheViewResolverTests.java b/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/servlet/view/MustacheViewResolverTests.java
index 61b25d0daa2..0483a8bd526 100644
--- a/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/servlet/view/MustacheViewResolverTests.java
+++ b/module/spring-boot-mustache/src/test/java/org/springframework/boot/mustache/servlet/view/MustacheViewResolverTests.java
@@ -16,6 +16,8 @@
 
 package org.springframework.boot.mustache.servlet.view;
 
+import java.util.Locale;
+
 import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -49,20 +51,21 @@ class MustacheViewResolverTests {
 
 	@Test
 	void resolveNonExistent() throws Exception {
-		assertThat(this.resolver.resolveViewName("bar", null)).isNull();
+		assertThat(this.resolver.resolveViewName("bar", Locale.ROOT)).isNull();
 	}
 
 	@Test
 	@WithResource(name = "template.html", content = "Hello {{World}}")
 	void resolveExisting() throws Exception {
-		assertThat(this.resolver.resolveViewName("template", null)).isNotNull();
+		assertThat(this.resolver.resolveViewName("template", Locale.ROOT)).isNotNull();
 	}
 
 	@Test
 	@WithResource(name = "template.html", content = "Hello {{World}}")
 	void setsContentType() throws Exception {
 		this.resolver.setContentType("application/octet-stream");
-		View view = this.resolver.resolveViewName("template", null);
+		View view = this.resolver.resolveViewName("template", Locale.ROOT);
+		assertThat(view).isNotNull();
 		Assertions.assertThat(view.getContentType()).isEqualTo("application/octet-stream");
 
 	}