Refine YAML type restriction error

Closes gh-21596

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/json/YamlJsonParser.java

@@ -26,6 +26,8 @@ import java.util.stream.Collectors;
 import org.yaml.snakeyaml.Yaml;
 import org.yaml.snakeyaml.constructor.Constructor;
 
+import org.springframework.util.Assert;
+
 /**
  * Thin wrapper to adapt Snake {@link Yaml} to {@link JsonParser}.
  *
@@ -63,7 +65,9 @@ public class YamlJsonParser extends AbstractJsonParser {
 
 		@Override
 		protected Class<?> getClassForName(String name) throws ClassNotFoundException {
-			return (SUPPORTED_TYPES.contains(name)) ? super.getClassForName(name) : null;
+			Assert.state(SUPPORTED_TYPES.contains(name),
+					() -> "Unsupported '" + name + "' type encountered in YAML document");
+			return super.getClassForName(name);
 		}
 
 	}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/json/YamlJsonParserTests.java

@@ -36,7 +36,8 @@ public class YamlJsonParserTests extends AbstractJsonParserTests {
 	@Test
 	public void customTypesAreNotLoaded() throws Exception {
 		assertThatExceptionOfType(ConstructorException.class)
-				.isThrownBy(() -> getParser().parseMap("{value: !!java.net.URL [\"http://localhost:9000/\"]}"));
+				.isThrownBy(() -> getParser().parseMap("{value: !!java.net.URL [\"http://localhost:9000/\"]}"))
+				.withCauseInstanceOf(IllegalStateException.class);
 	}
 
 }