diff --git a/xmlIO.c b/xmlIO.c
index 57312b9..f20c0fa 100644
--- a/xmlIO.c
+++ b/xmlIO.c
@@ -3401,12 +3401,18 @@
 		out->error = XML_IO_ENCODER;
 		return(-1);
 	    }
-            nbchars = ret >= 0 ? ret : 0;
+            if (out->writecallback)
+	        nbchars = xmlBufUse(out->conv);
+            else
+                nbchars = ret >= 0 ? ret : 0;
 	} else {
 	    ret = xmlBufAdd(out->buffer, (const xmlChar *) buf, chunk);
 	    if (ret != 0)
 	        return(-1);
-            nbchars = chunk;
+            if (out->writecallback)
+	        nbchars = xmlBufUse(out->buffer);
+            else
+                nbchars = chunk;
 	}
 	buf += chunk;
 	len -= chunk;
@@ -3593,13 +3599,19 @@
 		out->error = XML_IO_ENCODER;
 		return(-1);
 	    }
-            nbchars = ret >= 0 ? ret : 0;
+            if (out->writecallback)
+	        nbchars = xmlBufUse(out->conv);
+            else
+                nbchars = ret >= 0 ? ret : 0;
 	} else {
 	    ret = escaping(xmlBufEnd(out->buffer), &chunk, str, &cons);
 	    if ((ret < 0) || (chunk == 0)) /* chunk==0 => nothing done */
 	        return(-1);
             xmlBufAddLen(out->buffer, chunk);
-            nbchars = chunk;
+            if (out->writecallback)
+	        nbchars = xmlBufUse(out->buffer);
+            else
+                nbchars = chunk;
 	}
 	str += cons;
 	len -= cons;