diff --git a/parser.c b/parser.c
index f8a41a57..df79a7ca 100644
--- a/parser.c
+++ b/parser.c
@@ -2115,7 +2115,10 @@ static int spacePop(xmlParserCtxtPtr ctxt) {
 	xmlSHRINK (ctxt);
 
 static void xmlSHRINK (xmlParserCtxtPtr ctxt) {
-    xmlParserInputShrink(ctxt->input);
+    /* Don't shrink memory buffers. */
+    if ((ctxt->input->buf) &&
+        ((ctxt->input->buf->encoder) || (ctxt->input->buf->readcallback)))
+        xmlParserInputShrink(ctxt->input);
     if (*ctxt->input->cur == 0)
         xmlParserInputGrow(ctxt->input, INPUT_CHUNK);
 }
diff --git a/xmlreader.c b/xmlreader.c
index f367641f..f4ce22b1 100644
--- a/xmlreader.c
+++ b/xmlreader.c
@@ -840,7 +840,7 @@ xmlTextReaderPushData(xmlTextReaderPtr reader) {
      * Discard the consumed input when needed and possible
      */
     if (reader->mode == XML_TEXTREADER_MODE_INTERACTIVE) {
-        if (alloc != XML_BUFFER_ALLOC_IMMUTABLE) {
+        if (reader->input->readcallback != NULL) {
 	    if ((reader->cur >= 4096) &&
 		(xmlBufUse(inbuf) - reader->cur <= CHUNK_SIZE)) {
 		val = xmlBufShrink(inbuf, reader->cur);