diff --git a/panda/src/express/subStreamBuf.cxx b/panda/src/express/subStreamBuf.cxx
index e89a73d563..7f298fc5b3 100644
--- a/panda/src/express/subStreamBuf.cxx
+++ b/panda/src/express/subStreamBuf.cxx
@@ -291,6 +291,7 @@ underflow() {
         if (_end != (streampos)0) {
           _end = _cur;
         }
+        gbump(num_bytes);
         return EOF;
       }
 
diff --git a/panda/src/express/zStreamBuf.cxx b/panda/src/express/zStreamBuf.cxx
index c01c2fa931..8853712500 100644
--- a/panda/src/express/zStreamBuf.cxx
+++ b/panda/src/express/zStreamBuf.cxx
@@ -221,6 +221,7 @@ underflow() {
     if (read_count != num_bytes) {
       // Oops, we didn't read what we thought we would.
       if (read_count == 0) {
+        gbump(num_bytes);
         return EOF;
       }