diff --git a/alivepdf/src/org/alivepdf/images/PNGImage.as b/alivepdf/src/org/alivepdf/images/PNGImage.as
index 7116775..8059fca 100755
--- a/alivepdf/src/org/alivepdf/images/PNGImage.as
+++ b/alivepdf/src/org/alivepdf/images/PNGImage.as
@@ -81,7 +81,7 @@ package org.alivepdf.images
 					
 				} else if ( type == PNGImage.TRNS )
 				{
-					
+					throw new Error("PNG simple transparency mode is not supported");
 					
 				} else if ( type == PNGImage.IDAT )
 				{	
@@ -105,4 +105,4 @@ package org.alivepdf.images
 	    	return idataBytes;	
 	    }
 	}
-}
\ No newline at end of file
+}