diff --git a/iocore/eventsystem/P_IOBuffer.h b/iocore/eventsystem/P_IOBuffer.h
index a3193b60b00..1448f9a0caf 100644
--- a/iocore/eventsystem/P_IOBuffer.h
+++ b/iocore/eventsystem/P_IOBuffer.h
@@ -551,6 +551,7 @@ IOBufferReader::is_read_avail_more_than(int64_t size)
 TS_INLINE void
 IOBufferReader::consume(int64_t n)
 {
+  ink_assert(read_avail() >= n);
   start_offset += n;
   if (size_limit != INT64_MAX) {
     size_limit -= n;
diff --git a/plugins/multiplexer/fetcher.h b/plugins/multiplexer/fetcher.h
index 45e892f2bac..fe236142645 100644
--- a/plugins/multiplexer/fetcher.h
+++ b/plugins/multiplexer/fetcher.h
@@ -211,6 +211,8 @@ template <class T> struct HttpTransaction {
             self->t_.header(self->parser_.buffer_, self->parser_.location_);
             self->parsingHeaders_ = false;
           }
+          // Parsing headers will indirectly read from our reader. Update available accordingly.
+          available = TSIOBufferReaderAvail(self->in_->reader);
         }
         if (!self->parsingHeaders_) {
           if (self->chunkDecoder_ != NULL) {