diff --git a/tool/src/org/antlr/v4/parse/ANTLRLexer.g b/tool/src/org/antlr/v4/parse/ANTLRLexer.g
index 975c8d21b..71e03573b 100644
--- a/tool/src/org/antlr/v4/parse/ANTLRLexer.g
+++ b/tool/src/org/antlr/v4/parse/ANTLRLexer.g
@@ -656,8 +656,11 @@ ESC_SEQ
     	    | // An illegal escape seqeunce
     	      //
     	      {
-    	      	// TODO: Issue error message
-    	      	//
+                Token t = new CommonToken(input, state.type, state.channel, getCharIndex()-1, getCharIndex());
+                t.setText(t.getText());
+                t.setLine(input.getLine());
+                t.setCharPositionInLine(input.getCharPositionInLine()-1);
+                grammarError(ErrorType.INVALID_ESCAPE_SEQUENCE, t);
     	      }
         )
     ;
@@ -708,9 +711,12 @@ UNICODE_ESC
     	// Now check the digit count and issue an error if we need to
     	//
     	{
-    		if	(hCount != 4) {
-
-    			// TODO: Issue error message
+    		if (hCount != 4) {
+                Token t = new CommonToken(input, state.type, state.channel, getCharIndex()-3-hCount, getCharIndex()-1);
+                t.setText(t.getText());
+                t.setLine(input.getLine());
+                t.setCharPositionInLine(input.getCharPositionInLine()-hCount-2);
+                grammarError(ErrorType.INVALID_ESCAPE_SEQUENCE, t);
     		}
     	}
     ;
diff --git a/tool/src/org/antlr/v4/tool/ErrorType.java b/tool/src/org/antlr/v4/tool/ErrorType.java
index 451a03a41..f20f8f48c 100644
--- a/tool/src/org/antlr/v4/tool/ErrorType.java
+++ b/tool/src/org/antlr/v4/tool/ErrorType.java
@@ -809,6 +809,23 @@ public enum ErrorType {
 	 * @since 4.2
 	 */
 	UNKNOWN_LEXER_CONSTANT(155, "rule '<arg>' contains a lexer command with an unrecognized constant value; lexer interpreters may produce incorrect output", ErrorSeverity.WARNING),
+	/**
+	 * Compiler Error 156.
+	 *
+	 * <p>invalid escape sequence</p>
+	 *
+	 * <p>The grammar contains a string literal with an invalid escape sequence.</p>
+	 *
+	 * <p>The following rule produces this error.</p>
+	 *
+	 * <pre>
+	 * x : 'x';  // ok
+	 * y : '\u005Cu'; // error 156
+	 * </pre>
+	 *
+	 * @since 4.2.1
+	 */
+	INVALID_ESCAPE_SEQUENCE(156, "invalid escape sequence", ErrorSeverity.ERROR),
 
 	/*
 	 * Backward incompatibility errors
diff --git a/tool/test/org/antlr/v4/test/TestToolSyntaxErrors.java b/tool/test/org/antlr/v4/test/TestToolSyntaxErrors.java
index ee5b4e08b..ddf96d585 100644
--- a/tool/test/org/antlr/v4/test/TestToolSyntaxErrors.java
+++ b/tool/test/org/antlr/v4/test/TestToolSyntaxErrors.java
@@ -409,4 +409,46 @@ public class TestToolSyntaxErrors extends BaseTest {
 
 		super.testErrors(pair, true);
 	}
+
+	/**
+	 * This test ensures that the {@link ErrorType#INVALID_ESCAPE_SEQUENCE}
+	 * error is not reported for escape sequences that are known to be valid.
+	 */
+	@Test public void testValidEscapeSequences() {
+		String grammar =
+			"lexer grammar A;\n" +
+			"NORMAL_ESCAPE : '\\b \\t \\n \\f \\r \\\" \\' \\\\';\n" +
+			"UNICODE_ESCAPE : '\\u0001 \\u00A1 \\u00a1 \\uaaaa \\uAAAA';\n";
+		String expected =
+			"";
+
+		String[] pair = new String[] {
+			grammar,
+			expected
+		};
+
+		super.testErrors(pair, true);
+	}
+
+	/**
+	 * This is a regression test for antlr/antlr4#507 "NullPointerException When
+	 * Generating Code from Grammar".
+	 * https://github.com/antlr/antlr4/issues/507
+	 */
+	@Test public void testInvalidEscapeSequences() {
+		String grammar =
+			"lexer grammar A;\n" +
+			"RULE : 'Foo \\uAABG \\x \\u';\n";
+		String expected =
+			"error(" + ErrorType.INVALID_ESCAPE_SEQUENCE.code + "): A.g4:2:12: invalid escape sequence\n" +
+			"error(" + ErrorType.INVALID_ESCAPE_SEQUENCE.code + "): A.g4:2:19: invalid escape sequence\n" +
+			"error(" + ErrorType.INVALID_ESCAPE_SEQUENCE.code + "): A.g4:2:22: invalid escape sequence\n";
+
+		String[] pair = new String[] {
+			grammar,
+			expected
+		};
+
+		super.testErrors(pair, true);
+	}
 }