diff --git a/page.go b/page.go
index 479b243..322a43a 100644
--- a/page.go
+++ b/page.go
@@ -496,8 +496,10 @@ func (p Page) Content() (result Content) {
 
 		case "Q": // restore graphics state
 			n := len(gstack) - 1
-			g = gstack[n]
-			gstack = gstack[:n]
+			if n >= 0 {
+			    g = gstack[n]
+			    gstack = gstack[:n]
+			}
 
 		case "BT": // begin text (reset text matrix and line matrix)
 			g.Tm = ident