diff --git a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSyntaxErrorCode.java b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSyntaxErrorCode.java
index 62950abcf..3ee0fff04 100644
--- a/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSyntaxErrorCode.java
+++ b/org.eclipse.lemminx/src/main/java/org/eclipse/lemminx/extensions/contentmodel/participants/XMLSyntaxErrorCode.java
@@ -167,8 +167,23 @@ public static Range toLSPRange(XMLLocator location, XMLSyntaxErrorCode code, Obj
 			return XMLPositionUtility.selectChildEndTag(tag, offset, document);
 		}
 		case ContentIllegalInProlog: {
-			int endOffset = document.getText().indexOf("<");
 			int startOffset = offset + 1;
+			int endOffset = 0;
+			int errorOffset = offset + 1;
+			String text = document.getText();
+			int startPrologOffset = text.indexOf("<");
+			if (errorOffset < startPrologOffset) {
+				// Invalid content given before prolog. Prolog should be the first thing in the
+				// file if given.
+				startOffset = errorOffset;
+				endOffset = startPrologOffset;
+			} else {
+				// Invalid content given after prolog. Either root tag or comment should be
+				// present
+				int firstStartTagOffset = text.indexOf("<", errorOffset);
+				startOffset = errorOffset;
+				endOffset = firstStartTagOffset != -1 ? firstStartTagOffset : text.length();
+			}
 			return XMLPositionUtility.createRange(startOffset, endOffset, document);
 		}
 		case DashDashInComment: {
diff --git a/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSyntaxDiagnosticsTest.java b/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSyntaxDiagnosticsTest.java
index 64a1b0202..73915904a 100644
--- a/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSyntaxDiagnosticsTest.java
+++ b/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx/extensions/contentmodel/XMLSyntaxDiagnosticsTest.java
@@ -85,11 +85,35 @@ public void testAttributePrefixUnbound() throws Exception {
 	 * @throws Exception
 	 */
 	@Test
-	public void testContentIllegalInProlog() throws Exception {
+	public void testBeforeContentIllegalInProlog() throws Exception {
 		String xml = " ab?<xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>";
 		testDiagnosticsFor(xml, d(0, 1, 0, 4, XMLSyntaxErrorCode.ContentIllegalInProlog));
 	}
 
+	/**
+	 * ContentIllegalInProlog tests
+	 * 
+	 * @see https://wiki.xmldation.com/Support/Validator/ContentIllegalInProlog
+	 * @throws Exception
+	 */
+	@Test
+	public void testAfterContentIllegalInProlog() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>ab\ncd";
+		testDiagnosticsFor(xml, d(0, 54, 1, 2, XMLSyntaxErrorCode.ContentIllegalInProlog));
+	}
+
+	/**
+	 * ContentIllegalInProlog tests
+	 * 
+	 * @see https://wiki.xmldation.com/Support/Validator/ContentIllegalInProlog
+	 * @throws Exception
+	 */
+	@Test
+	public void testAfterContentIllegalInProlog2() throws Exception {
+		String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?>ab\ncd<root>";
+		testDiagnosticsFor(xml, d(0, 54, 1, 2, XMLSyntaxErrorCode.ContentIllegalInProlog));
+	}
+
 	@Test
 	public void testEncodingUTF_16() throws Exception {
 		String xml = "<?xml version=\"1.0\" encoding=\"UTF-16\" standalone=\"no\"?><root />";