diff --git a/NEWS b/NEWS
index 33697acf622..0472345f2ab 100644
--- a/NEWS
+++ b/NEWS
@@ -65,6 +65,9 @@ PHP                                                                        NEWS
   . Fixed bug GH-16433 (Large values for openssl_csr_sign() $days overflow).
     (cmb)
 
+- Phar:
+  . Fixed bug GH-16406 (Assertion failure in ext/phar/phar.c:2808). (nielsdos)
+
 - PHPDBG:
   . Fixed bug GH-16174 (Empty string is an invalid expression for ev). (cmb)
 
diff --git a/ext/phar/phar_object.c b/ext/phar/phar_object.c
index 4a2f8726191..25f0d7d9fa6 100644
--- a/ext/phar/phar_object.c
+++ b/ext/phar/phar_object.c
@@ -2308,6 +2308,9 @@ no_copy:
 			newentry.tar_type = (entry->is_dir ? TAR_DIR : TAR_FILE);
 		}
 
+		/* The header offset is only used for unmodified zips.
+		 * Once modified, phar_zip_changed_apply_int() will update the header_offset. */
+		newentry.header_offset = 0;
 		newentry.is_modified = 1;
 		newentry.phar = phar;
 		newentry.old_flags = newentry.flags & ~PHAR_ENT_COMPRESSION_MASK; /* remove compression from old_flags */
diff --git a/ext/phar/tests/gh16406.phpt b/ext/phar/tests/gh16406.phpt
new file mode 100644
index 00000000000..6df5fd3aab6
--- /dev/null
+++ b/ext/phar/tests/gh16406.phpt
@@ -0,0 +1,35 @@
+--TEST--
+GH-16406 (Assertion failure in ext/phar/phar.c:2808)
+--EXTENSIONS--
+phar
+zlib
+--INI--
+phar.readonly=0
+phar.require_hash=0
+--FILE--
+<?php
+$fname = __DIR__ . '/gh16406.phar';
+@unlink($fname . '.tar');
+@unlink($fname . '.gz');
+@unlink($fname);
+$file = '<?php __HALT_COMPILER(); ?>';
+$files['b'] = 'b';
+$files['c'] = 'c';
+include __DIR__.'/files/phar_test.inc';
+$phar = new Phar($fname);
+$phar->compressFiles(Phar::GZ);
+$phar = $phar->convertToExecutable(Phar::TAR);
+$phar = $phar->convertToExecutable(Phar::PHAR, Phar::GZ);
+var_dump($phar['b']->openFile()->fread(4096));
+var_dump($phar['c']->openFile()->fread(4096));
+?>
+--CLEAN--
+<?php
+$fname = __DIR__ . '/gh16406.phar';
+@unlink($fname . '.tar');
+@unlink($fname . '.gz');
+@unlink($fname);
+?>
+--EXPECT--
+string(1) "b"
+string(1) "c"