diff --git a/spring-web/src/main/java/org/springframework/web/util/HierarchicalUriComponents.java b/spring-web/src/main/java/org/springframework/web/util/HierarchicalUriComponents.java
index a037452b606..f26c2b737fd 100644
--- a/spring-web/src/main/java/org/springframework/web/util/HierarchicalUriComponents.java
+++ b/spring-web/src/main/java/org/springframework/web/util/HierarchicalUriComponents.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -46,6 +46,7 @@ import org.springframework.util.StringUtils;
  * @author Juergen Hoeller
  * @author Rossen Stoyanchev
  * @author Phillip Webb
+ * @author Sam Brannen
  * @since 3.1.3
  * @see <a href="https://tools.ietf.org/html/rfc3986#section-1.2.3">Hierarchical URIs</a>
  */
@@ -192,7 +193,12 @@ final class HierarchicalUriComponents extends UriComponents {
 			throw new IllegalStateException(
 					"The port contains a URI variable but has not been expanded yet: " + this.port);
 		}
-		return Integer.parseInt(this.port);
+		try {
+			return Integer.parseInt(this.port);
+		}
+		catch (NumberFormatException ex) {
+			throw new IllegalStateException("The port must be an integer: " + this.port);
+		}
 	}
 
 	@Override
diff --git a/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java b/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java
index 69a7af3112d..1d3c06d8f7f 100644
--- a/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java
+++ b/spring-web/src/test/java/org/springframework/web/util/UriComponentsBuilderTests.java
@@ -40,7 +40,7 @@ import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
-import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
  * Unit tests for {@link UriComponentsBuilder}.
@@ -1314,11 +1314,13 @@ class UriComponentsBuilderTests {
 
 	@Test
 	void verifyInvalidPort() {
-		String url = "http://localhost:port/path";
-		assertThatThrownBy(() -> UriComponentsBuilder.fromUriString(url).build().toUri())
-				.isInstanceOf(NumberFormatException.class);
-		assertThatThrownBy(() -> UriComponentsBuilder.fromHttpUrl(url).build().toUri())
-				.isInstanceOf(NumberFormatException.class);
+		String url = "http://localhost:XXX/path";
+		assertThatIllegalStateException()
+				.isThrownBy(() -> UriComponentsBuilder.fromUriString(url).build().toUri())
+				.withMessage("The port must be an integer: XXX");
+		assertThatIllegalStateException()
+				.isThrownBy(() -> UriComponentsBuilder.fromHttpUrl(url).build().toUri())
+				.withMessage("The port must be an integer: XXX");
 	}
 
 	@Test  // gh-27039
diff --git a/spring-web/src/test/java/org/springframework/web/util/UriComponentsTests.java b/spring-web/src/test/java/org/springframework/web/util/UriComponentsTests.java
index 541cf18042b..7bbb2c34299 100644
--- a/spring-web/src/test/java/org/springframework/web/util/UriComponentsTests.java
+++ b/spring-web/src/test/java/org/springframework/web/util/UriComponentsTests.java
@@ -30,6 +30,7 @@ import org.junit.jupiter.api.Test;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+import static org.springframework.web.util.UriComponentsBuilder.fromHttpUrl;
 import static org.springframework.web.util.UriComponentsBuilder.fromUriString;
 
 /**
@@ -151,6 +152,29 @@ class UriComponentsTests {
 		assertThat(uri4.toUriString()).isEqualTo("https://example.com:8080/bar");
 	}
 
+	@Test  // gh-28521
+	void invalidPort() {
+		assertExceptionsForInvalidPort(fromUriString("https://example.com:XXX/bar").build());
+		assertExceptionsForInvalidPort(fromUriString("https://example.com/bar").port("XXX").build());
+		assertExceptionsForInvalidPort(fromHttpUrl("https://example.com:XXX/bar").build());
+		assertExceptionsForInvalidPort(fromHttpUrl("https://example.com/bar").port("XXX").build());
+	}
+
+	private void assertExceptionsForInvalidPort(UriComponents uriComponents) {
+		assertThatIllegalStateException()
+			.isThrownBy(uriComponents::getPort)
+			.withMessage("The port must be an integer: XXX");
+		assertThatIllegalStateException()
+			.isThrownBy(uriComponents::toUri)
+			.withMessage("The port must be an integer: XXX");
+		assertThatIllegalStateException()
+			.isThrownBy(uriComponents::toUriString)
+			.withMessage("The port must be an integer: XXX");
+		assertThatIllegalStateException()
+			.isThrownBy(uriComponents::toString)
+			.withMessage("The port must be an integer: XXX");
+	}
+
 	@Test
 	void expandEncoded() {
 		assertThatIllegalStateException().isThrownBy(() ->