diff --git a/astroquery/heasarc/core.py b/astroquery/heasarc/core.py
index 56f9b9397a..4fff8fa236 100644
--- a/astroquery/heasarc/core.py
+++ b/astroquery/heasarc/core.py
@@ -154,10 +154,10 @@ def tables(self, *, master=False, keywords=None):
         `~astropy.table.Table` with columns: name, description
 
         """
-        if isinstance(keywords, str):
+        if not isinstance(keywords, list):
             keywords = [keywords]
-            if not all([isinstance(wrd, str) for wrd in keywords]):
-                raise ValueError('non-str found in keywords elements')
+        if not all([isinstance(wrd, str) for wrd in keywords]):
+            raise ValueError('non-str found in keywords elements')
 
         # use 'mast' to include both 'master' and 'mastr'
         names, desc = [], []