diff --git a/src/gd.c b/src/gd.c
index 574368c..0be8aad 100644
--- a/src/gd.c
+++ b/src/gd.c
@@ -919,7 +919,9 @@ BGD_DECLARE(void) gdImageColorTransparent (gdImagePtr im, int color)
 	if (color >= gdMaxColors) {
 		return;
 	}
-	im->alpha[im->transparent] = gdAlphaOpaque;
+	if (im->transparent != -1) {
+		im->alpha[im->transparent] = gdAlphaOpaque;
+	}
 	im->alpha[color] = gdAlphaTransparent;
 	im->transparent = color;
 }