diff --git a/tool/src/org/antlr/v4/tool/ErrorType.java b/tool/src/org/antlr/v4/tool/ErrorType.java
index e3869cea8..ab23599ae 100644
--- a/tool/src/org/antlr/v4/tool/ErrorType.java
+++ b/tool/src/org/antlr/v4/tool/ErrorType.java
@@ -150,6 +150,7 @@ public enum ErrorType {
 //	ALIAS_REASSIGNMENT(127, "token literal <arg> aliased to new token name <arg2>", ErrorSeverity.WARNING),
 	ATTRIBUTE_IN_LEXER_ACTION(128, "attribute references not allowed in lexer actions: $<arg>", ErrorSeverity.ERROR),
 	WILDCARD_IN_PARSER(129, "wildcard '.' not allowed in parsers", ErrorSeverity.ERROR),
+	LABEL_BLOCK_NOT_A_SET(130, "label <arg> assigned to a block which is not a set", ErrorSeverity.ERROR),
 
 	/** Documentation comment is unterminated */
     //UNTERMINATED_DOC_COMMENT(, "", ErrorSeverity.ERROR),
diff --git a/tool/test/org/antlr/v4/test/TestBasicSemanticErrors.java b/tool/test/org/antlr/v4/test/TestBasicSemanticErrors.java
index 524693ace..3d29aa954 100644
--- a/tool/test/org/antlr/v4/test/TestBasicSemanticErrors.java
+++ b/tool/test/org/antlr/v4/test/TestBasicSemanticErrors.java
@@ -66,4 +66,23 @@ public class TestBasicSemanticErrors extends BaseTest {
     };
 
 	@Test public void testU() { super.testErrors(U, false); }
+
+	/**
+	 * Regression test for #25 "Don't allow labels on not token set subrules".
+	 * https://github.com/antlr/antlr4/issues/25
+	 */
+	@Test
+	public void testIllegalNonSetLabel() throws Exception {
+		String grammar =
+			"grammar T;\n" +
+			"ss : op=('=' | '+=' | expr) EOF;\n" +
+			"expr : '=' '=';\n" +
+			"";
+
+		String expected =
+			"error(130): T.g4:2:5: label op assigned to a block which is not a set\n";
+
+		testErrors(new String[] { grammar, expected }, false);
+	}
+
 }