--- src/lib/EtonyekDocument.cpp
+++ src/lib/EtonyekDocument.cpp
@@ -179,7 +181,7 @@
     info.input->seek(0, RVNG_SEEK_SET);
   }
 
-  assert(isGzipped == false);
+  assert(bool(!isGzipped));
 
   return probeXMLImpl(info.input, probe, type, info);
 }