diff --git a/python/obitools3/commands/grep.pyx b/python/obitools3/commands/grep.pyx
index a32995d..78bb516 100644
--- a/python/obitools3/commands/grep.pyx
+++ b/python/obitools3/commands/grep.pyx
@@ -207,9 +207,12 @@ def Filter_generator(options, tax_filter):
                    )
              
         if good and predicates:
-            good = (reduce(lambda bint x, bint y: x and y,
-                    (bool(eval(p, loc_env, line))
-                    for p in predicates), True))
+            try:
+                good = (reduce(lambda bint x, bint y: x and y,
+                        (bool(eval(p, loc_env, line))
+                        for p in predicates), True))
+            except TypeError:
+                raise Exception("Try replacing strings in python expression with bytes (by putting 'b' in front of strings, e.g. b'example')")
 
         if good and lmin:
             good = len(line) >= lmin