diff --git a/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java b/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java
index be865004ad..f5c8163713 100644
--- a/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java
+++ b/spring-web/src/main/java/org/springframework/web/util/UriComponentsBuilder.java
@@ -236,13 +236,16 @@ public class UriComponentsBuilder implements UriBuilder, Cloneable {
 			}
 			builder.scheme(scheme);
 			if (opaque) {
-				String ssp = uri.substring(scheme.length()).substring(1);
+				String ssp = uri.substring(scheme.length() + 1);
 				if (StringUtils.hasLength(fragment)) {
 					ssp = ssp.substring(0, ssp.length() - (fragment.length() + 1));
 				}
 				builder.schemeSpecificPart(ssp);
 			}
 			else {
+				if (StringUtils.hasLength(scheme) && !StringUtils.hasLength(host)) {
+					throw new IllegalArgumentException("[" + uri + "] is not a valid URI");
+				}
 				builder.userInfo(userInfo);
 				builder.host(host);
 				if (StringUtils.hasLength(port)) {
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 d19c2740a2..34cebf5e0d 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
@@ -219,6 +219,12 @@ class UriComponentsBuilderTests {
 		assertThat(resultIPv4compatible.getHost()).isEqualTo("[::192.168.1.1]");
 	}
 
+	@Test
+	void fromUriStringInvalidIPv6Host() {
+		assertThatIllegalArgumentException().isThrownBy(() ->
+				UriComponentsBuilder.fromUriString("http://[1abc:2abc:3abc::5ABC:6abc:8080/resource"));
+	}
+
 	@Test  // SPR-11970
 	void fromUriStringNoPathWithReservedCharInQuery() {
 		UriComponents result = UriComponentsBuilder.fromUriString("https://example.com?foo=bar@baz").build();