diff --git a/include/qpdf/QPDFObjGen.hh b/include/qpdf/QPDFObjGen.hh
index e0834a9..a1829b0 100644
--- a/include/qpdf/QPDFObjGen.hh
+++ b/include/qpdf/QPDFObjGen.hh
@@ -38,7 +38,7 @@ class QPDFObjGen
     {
     }
     QPDF_DLL
-    QPDFObjGen(int obj, int gen) :
+    explicit QPDFObjGen(int obj, int gen = 0) :
         obj(obj),
         gen(gen)
     {