diff --git a/src/calibre/utils/podofo/output.cpp b/src/calibre/utils/podofo/output.cpp
index 015949502a..d72fd316fe 100644
--- a/src/calibre/utils/podofo/output.cpp
+++ b/src/calibre/utils/podofo/output.cpp
@@ -40,7 +40,7 @@ class OutputDevice : public PdfOutputDevice {
             char *buf;
             int res, len=1024;
             while(true) {
-                buf = new (std::nothrow) char[len];
+                buf = new (std::nothrow) char[len+1];
                 if (buf == NULL) { PyErr_NoMemory(); throw pyerr(); }
                 res = vsnprintf(buf, len, pszFormat, args);
                 delete[] buf;