diff --git a/sherpa/astro/xspec/__init__.py b/sherpa/astro/xspec/__init__.py
index 5fed743a78..155d8a6213 100644
--- a/sherpa/astro/xspec/__init__.py
+++ b/sherpa/astro/xspec/__init__.py
@@ -1433,11 +1433,17 @@ def calc(self, p, *args, **kwargs):
             #   - model class
             #   - model name
             #   - parameter list
+            #   - any kwargs
             #
             msg = f"{ve}: {self.type}.{self.name}"
-            for par, val in zip(self.pars, args[0]):
+            for par, val in zip(self.pars, p):
                 msg += f" {par.name}={val}"
 
+            if kwargs:
+                msg += " KEYWORDS "
+                for k, v in kwargs.items():
+                    msg += f"{k}={v}"
+
             raise ValueError(msg) from None
 
         # This check is being skipped in the 4.8.0 release as it