diff --git a/pcse/base/parameter_providers.py b/pcse/base/parameter_providers.py
index f7f273ea..ef4545c5 100644
--- a/pcse/base/parameter_providers.py
+++ b/pcse/base/parameter_providers.py
@@ -6,18 +6,15 @@
 In general these classes are not to be used directly, but are to be subclassed
 when creating PCSE simulation units.
 """
-import types
+import sys
 import logging
-from datetime import date
-import pickle
-from collections import Counter, MutableMapping
+from collections import Counter
+if sys.version_info > (3, 8):
+    from collections.abc import MutableMapping
+else:
+    from collections import MutableMapping
 
-from ..traitlets import (HasTraits, List, Float, Int, Instance, Dict, Bool, All)
-from ..pydispatch import dispatcher
-from ..util import Afgen
 from .. import exceptions as exc
-from ..settings import settings
-from .variablekiosk import VariableKiosk
 
 
 class ParameterProvider(MutableMapping):
@@ -43,7 +40,6 @@ class ParameterProvider(MutableMapping):
     _cropdata = dict()
     _timerdata = dict()
     _override = dict()
-    _unique_parameters = list()
     _iter = 0  # Counter for iterator
     _ncrops_activated = 0  # Counts the number of times `set_crop_type()` has been called.
 
@@ -136,7 +132,7 @@ def set_override(self, varname, value, check=True):
             if varname in self:
                 self._override[varname] = value
             else:
-                msg = "Cannot override '%s', parameter does not exist." % varname
+                msg = "Cannot override '%s', parameter does not already exist." % varname
                 raise exc.PCSEError(msg)
         else:
             self._override[varname] = value
diff --git a/pcse/fileinput/yaml_cropdataprovider.py b/pcse/fileinput/yaml_cropdataprovider.py
index 8157cfbe..6b268bf2 100644
--- a/pcse/fileinput/yaml_cropdataprovider.py
+++ b/pcse/fileinput/yaml_cropdataprovider.py
@@ -227,6 +227,10 @@ def _get_yaml_files(self, fpath):
             raise exc.PCSEError(msg)
         crop_names = yaml.safe_load(open(fname))["available_crops"]
         crop_yaml_fnames = {crop: os.path.join(fpath, crop + ".yaml") for crop in crop_names}
+        for crop, fname in crop_yaml_fnames.items():
+            if not os.path.exists(fname):
+                msg = f"Cannot find yaml file for crop '{crop}': {fname}"
+                raise RuntimeError(msg)
         return crop_yaml_fnames
 
     def set_active_crop(self, crop_name, variety_name):
diff --git a/pcse/util.py b/pcse/util.py
index d8d8e724..9046f806 100644
--- a/pcse/util.py
+++ b/pcse/util.py
@@ -14,7 +14,10 @@
 from bisect import bisect_left
 import textwrap
 import sqlite3
-from collections import Iterable
+if sys.version_info > (3,8):
+    from collections.abc import Iterable
+else:
+    from collections import Iterable
 
 from . import exceptions as exc
 from .traitlets import TraitType
diff --git a/requirements.txt b/requirements.txt
index b8068969..0d2c832d 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,6 @@
-SQLAlchemy>=0.8.0
-numpy>=1.6.0
-PyYAML>=3.11
+SQLAlchemy>=1.3,<2.0
+PyYAML>=5.1
 openpyxl>=3.0
 requests>=2.0.0
-pandas>=0.20
+pandas>=0.25
 traitlets-pcse==5.0.0.dev
diff --git a/setup.py b/setup.py
index 2753a39d..453a72da 100644
--- a/setup.py
+++ b/setup.py
@@ -37,7 +37,7 @@ def read(*filenames, **kwargs):
     download_url='http://github.com/ajwdewit/pcse/tarball/'+VERSION,
     license='EUPL',
     author=AUTHOR,
-    install_requires=['SQLAlchemy>=1.3.0',
+    install_requires=['SQLAlchemy>=1.3.0, <2.0',
                       'PyYAML>=5.1',
                       'openpyxl>=3.0.0',
                       'requests>=2.0.0',