diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index b6bf9e90ffc..ffec5e84373 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -230,6 +230,8 @@ PHP_RINIT_FUNCTION(zlib)
 	ZLIBG(ob_gzhandler_status) = 0;
 	ZLIBG(ob_gzip_coding) = 0;
 	if (chunk_size) {
+		if (chunk_size == 1)
+			chunk_size = 0; /* use the default size */
 		php_enable_output_compression(chunk_size TSRMLS_CC);
 	}
 	return SUCCESS;
diff --git a/main/output.c b/main/output.c
index c55286e3857..17a6491063a 100644
--- a/main/output.c
+++ b/main/output.c
@@ -132,6 +132,8 @@ PHPAPI int php_start_ob_buffer(zval *output_handler, uint chunk_size, zend_bool
 	if (chunk_size) {
 		initial_size = (chunk_size*3/2);
 		block_size = chunk_size/2;
+		if (block_size == 0)
+			block_size = 1;
 	} else {
 		initial_size = 40*1024;
 		block_size = 10*1024;