diff --git a/common/rfb/PixelFormat.cxx b/common/rfb/PixelFormat.cxx
index 4221e44..e8cb1a2 100644
--- a/common/rfb/PixelFormat.cxx
+++ b/common/rfb/PixelFormat.cxx
@@ -102,6 +102,8 @@
 
 bool PixelFormat::is888(void) const
 {
+  if (!trueColour)
+    return false;
   if (bpp != 32)
     return false;
   if (depth != 24)