diff -rupN poppler-0.73.0/utils/pdfunite.cc poppler-0.73.0-new/utils/pdfunite.cc
--- poppler-0.73.0/utils/pdfunite.cc	2019-01-07 22:15:12.000000000 +0100
+++ poppler-0.73.0-new/utils/pdfunite.cc	2019-02-20 19:06:08.067874556 +0100
@@ -165,7 +165,7 @@ int main (int argc, char *argv[])
   for (i = 1; i < argc - 1; i++) {
     GooString *gfileName = new GooString(argv[i]);
     PDFDoc *doc = new PDFDoc(gfileName, nullptr, nullptr, nullptr);
-    if (doc->isOk() && !doc->isEncrypted()) {
+    if (doc->isOk() && !doc->isEncrypted() && doc->getXRef()->getCatalog().isDict()) {
       docs.push_back(doc);
       if (doc->getPDFMajorVersion() > majorVersion) {
         majorVersion = doc->getPDFMajorVersion();
@@ -175,9 +175,12 @@ int main (int argc, char *argv[])
           minorVersion = doc->getPDFMinorVersion();
         }
       }
-    } else if (doc->isOk()) {
+    } else if (doc->isEncrypted()) {
       error(errUnimplemented, -1, "Could not merge encrypted files ('{0:s}')", argv[i]);
       return -1;
+    } else if (!doc->getXRef()->getCatalog().isDict()) {
+      error(errSyntaxError, -1, "Catalog is not a dictionary ('{0:s}')", argv[i]);
+      return -1;
     } else {
       error(errSyntaxError, -1, "Could not merge damaged documents ('{0:s}')", argv[i]);
       return -1;