diff --git a/src/ai_models/inputs/opendata.py b/src/ai_models/inputs/opendata.py
index 9c3a1d9..16634c0 100644
--- a/src/ai_models/inputs/opendata.py
+++ b/src/ai_models/inputs/opendata.py
@@ -62,12 +62,12 @@ def _adjust(self, kwargs):
 
         if interp:
             logging.debug("Interpolating from %s to %s", source, grid)
-            return (Interpolate(grid, source), source)
+            return Interpolate(grid, source)
         else:
-            return (lambda x: x, source)
+            return lambda x: x
 
     def pl_load_source(self, **kwargs):
-        pproc, _ = self._adjust(kwargs)
+        pproc = self._adjust(kwargs)
         kwargs["levtype"] = "pl"
         request = kwargs.copy()
 
@@ -75,7 +75,7 @@ def pl_load_source(self, **kwargs):
         assert isinstance(param, (list, tuple))
 
         if "z" in param:
-            logging.warning("Parameter 'z' on pressure levels is not available in open data, using 'gh' instead")
+            logging.warning("Parameter 'z' on pressure levels is not available in ECMWF open data, using 'gh' instead")
             param = list(param)
             param.remove("z")
             if "gh" not in param:
@@ -87,7 +87,7 @@ def pl_load_source(self, **kwargs):
         return self.check_pl(pproc(ekd.from_source("ecmwf-open-data", **kwargs)), request)
 
     def sfc_load_source(self, **kwargs):
-        pproc, resol = self._adjust(kwargs)
+        pproc = self._adjust(kwargs)
         kwargs["levtype"] = "sfc"
         request = kwargs.copy()
 
@@ -107,18 +107,18 @@ def sfc_load_source(self, **kwargs):
             if len(constant_params) == 1:
                 logging.warning(
                     f"Single level parameter '{constant_params[0]}' is"
-                    " not available in open data, using constants.grib2 instead"
+                    " not available in ECMWF open data, using constants.grib2 instead"
                 )
             else:
                 logging.warning(
                     f"Single level parameters {constant_params} are"
-                    " not available in open data, using constants.grib2 instead"
+                    " not available in ECMWF open data, using constants.grib2 instead"
                 )
             constants = []
 
             cachedir = os.path.expanduser("~/.cache/ai-models")
-            constant_url = CONSTANTS_URL.format(resol=resol)
-            basename = os.path.basename(constant_url)
+            constants_url = CONSTANTS_URL.format(resol=request["resol"])
+            basename = os.path.basename(constants_url)
 
             if not os.path.exists(cachedir):
                 os.makedirs(cachedir)
@@ -126,8 +126,8 @@ def sfc_load_source(self, **kwargs):
             path = os.path.join(cachedir, basename)
 
             if not os.path.exists(path):
-                logging.info("Downloading %s to %s", constant_url, path)
-                download(constant_url, path + ".tmp")
+                logging.info("Downloading %s to %s", constants_url, path)
+                download(constants_url, path + ".tmp")
                 os.rename(path + ".tmp", path)
 
             ds = ekd.from_source("file", path)
@@ -191,6 +191,10 @@ def check_ml(self, ds, request):
 
     def _check(self, ds, what, request, *keys):
 
+        def _(p):
+            if len(p) == 1:
+                return p[0]
+
         expected = set()
         for p in itertools.product(*[request[key] for key in keys]):
             expected.add(p)
@@ -201,8 +205,14 @@ def _check(self, ds, what, request, *keys):
 
         missing = expected - found
         if missing:
-            raise ValueError(f"The following {what} parameters {missing} are not available in open data")
+            missing = [_(p) for p in missing]
+            if len(missing) == 1:
+                raise ValueError(f"The following {what} parameter '{missing[0]}' is not available in ECMWF open data")
+            raise ValueError(f"The following {what} parameters {missing} are not available in ECMWF open data")
 
         extra = found - expected
         if extra:
-            raise ValueError(f"Unexpected {what} parameters {extra} from open data")
+            extra = [_(p) for p in extra]
+            if len(extra) == 1:
+                raise ValueError(f"Unexpected {what} parameter '{extra[0]}' from ECMWF open data")
+            raise ValueError(f"Unexpected {what} parameters {extra} from ECMWF open data")