diff --git a/pdfrw/pdfwriter.py b/pdfrw/pdfwriter.py
index 3c887ba..69b4d24 100755
--- a/pdfrw/pdfwriter.py
+++ b/pdfrw/pdfwriter.py
@@ -151,6 +151,8 @@ def format_obj(obj):
                     return result
                 obj = (PdfArray, PdfDict)[isinstance(obj, dict)](obj)
                 continue
+            elif isinstance(obj, bool):
+                return 'true' if obj else 'false'
 
             # We assume that an object with an indirect
             # attribute knows how to represent itself to us.