diff --git a/libqpdf/QPDF_Dictionary.cc b/libqpdf/QPDF_Dictionary.cc
index df83632c..942e6d9e 100644
--- a/libqpdf/QPDF_Dictionary.cc
+++ b/libqpdf/QPDF_Dictionary.cc
@@ -122,7 +122,9 @@ void
 QPDF_Dictionary::replaceKey(std::string const& key, QPDFObjectHandle value)
 {
     if (value.isNull() && !value.isIndirect()) {
-        // The PDF spec doesn't distinguish between keys with null values and missing keys.
+        // The PDF spec doesn't distinguish between keys with null values and missing keys. Allow
+        // indirect nulls which are equivalent to a dangling reference, which is permitted by the
+        // spec.
         removeKey(key);
     } else {
         // add or replace value