diff --git a/fred.py b/fred.py
index 3112599..d4ad47e 100644
--- a/fred.py
+++ b/fred.py
@@ -66,6 +66,8 @@ def toGrid(input, parser=None):
         FileNotFoundError: If the file cannot be found (if input is a file path).
         ValueError: If the parser function is invalid.
     """
+    if parser is not None and not callable(parser):
+        raise ValueError("The parser must be a callable function.")
 
     grid = []
     try: