diff --git a/src/main/required/org/luaj/vm3/lib/StringLib.java b/src/main/required/org/luaj/vm3/lib/StringLib.java
index 317d68920..afde5979b 100644
--- a/src/main/required/org/luaj/vm3/lib/StringLib.java
+++ b/src/main/required/org/luaj/vm3/lib/StringLib.java
@@ -1155,7 +1155,7 @@ public class StringLib extends TwoArgFunction {
 			if ( poff == plen || poff + 1 == plen ) {
 				error( "unbalanced pattern" );
 			}
-			if ( s.luaByte( soff ) != p.luaByte( poff ) )
+			if ( soff >= s.length() || s.luaByte( soff ) != p.luaByte( poff ) )
 				return -1;
 			else {
 				int b = p.luaByte( poff );