diff --git a/archivebox/config/__init__.py b/archivebox/config/__init__.py
index c661a1d6..c98c4668 100644
--- a/archivebox/config/__init__.py
+++ b/archivebox/config/__init__.py
@@ -301,7 +301,7 @@ DERIVED_CONFIG_DEFAULTS: ConfigDefaultDict = {
     'READABILITY_VERSION':      {'default': lambda c: bin_version(c['READABILITY_BINARY']) if c['USE_READABILITY'] else None},
 
     'USE_MERCURY':              {'default': lambda c: c['USE_MERCURY'] and c['SAVE_MERCURY']},
-    'MERCURY_VERSION':          {'default': lambda c: bin_version(c['MERCURY_BINARY']) if c['USE_MERCURY'] else None},
+    'MERCURY_VERSION':          {'default': lambda c: '1.0.0' if (c['USE_MERCURY'] and c['MERCURY_BINARY']) else None},  # mercury is unversioned
 
     'USE_GIT':                  {'default': lambda c: c['USE_GIT'] and c['SAVE_GIT']},
     'GIT_VERSION':              {'default': lambda c: bin_version(c['GIT_BINARY']) if c['USE_GIT'] else None},