diff --git a/src/calibre/ebooks/conversion/plugins/txt_input.py b/src/calibre/ebooks/conversion/plugins/txt_input.py
index 82fae5d53e..682364d7c0 100644
--- a/src/calibre/ebooks/conversion/plugins/txt_input.py
+++ b/src/calibre/ebooks/conversion/plugins/txt_input.py
@@ -114,7 +114,7 @@ class TXTInput(InputFormatPlugin):
         for img in root.xpath('//img[@src]'):
             src = img.get('src')
             prefix = src.split(':', 1)[0].lower()
-            if prefix not in ('file', 'http', 'https', 'ftp') and not os.path.isabs(src):
+            if src and prefix not in ('file', 'http', 'https', 'ftp') and not os.path.isabs(src):
                 src = os.path.join(base_dir, src)
                 if os.path.isfile(src) and os.access(src, os.R_OK):
                     with open(src, 'rb') as f: