diff --git a/src/calibre/ebooks/conversion/plugins/txt_input.py b/src/calibre/ebooks/conversion/plugins/txt_input.py
index dd2b82cfde..82fae5d53e 100644
--- a/src/calibre/ebooks/conversion/plugins/txt_input.py
+++ b/src/calibre/ebooks/conversion/plugins/txt_input.py
@@ -116,7 +116,7 @@ class TXTInput(InputFormatPlugin):
             prefix = src.split(':', 1)[0].lower()
             if prefix not in ('file', 'http', 'https', 'ftp') and not os.path.isabs(src):
                 src = os.path.join(base_dir, src)
-                if os.access(src, os.R_OK):
+                if os.path.isfile(src) and os.access(src, os.R_OK):
                     with open(src, 'rb') as f:
                         data = f.read()
                     f = self.shift_file(os.path.basename(src), data)