diff --git a/dll/win32/usp10/shape.c b/dll/win32/usp10/shape.c
index 3827000122a75a76d959238d4ae59f3aece4ede0..af766b8d5e649f516c1a6c836536a0482611e25a 100644
--- a/dll/win32/usp10/shape.c
+++ b/dll/win32/usp10/shape.c
@@ -3152,7 +3152,7 @@ static void ShapeCharGlyphProp_Thai( HDC hdc, ScriptCache *psc, SCRIPT_ANALYSIS
     /* Do not allow justification between marks and their base */
     for (i = 0; i < cGlyphs; i++)
     {
-        if (!pGlyphProp[i].sva.fClusterStart)
+        if (!pGlyphProp[i].sva.fClusterStart && (i-dirL) >= 0)
             pGlyphProp[i-dirL].sva.uJustification = SCRIPT_JUSTIFY_NONE;
     }
 }