diff --git a/collectfast/strategies/base.py b/collectfast/strategies/base.py
index 833de96..6c4e496 100644
--- a/collectfast/strategies/base.py
+++ b/collectfast/strategies/base.py
@@ -150,9 +150,9 @@ def load_strategy(klass: Union[str, type, object]) -> Type[Strategy]:
 
 
 def _resolves_to_subclass(subclass_ref: str, superclass_ref: str) -> bool:
-    subclass = locate(subclass_ref)
-    assert isinstance(subclass, type)
     try:
+        subclass = locate(subclass_ref)
+        assert isinstance(subclass, type)
         superclass = locate(superclass_ref)
         assert isinstance(superclass, type)
     except (ImportError, AssertionError) as e:
@@ -176,5 +176,6 @@ def guess_strategy(storage: str) -> str:
     if _resolves_to_subclass(storage, _BOTO3_STORAGE):
         return _BOTO3_STRATEGY
     raise ImproperlyConfigured(
-        "No strategy configured, please make sure COLLECTFAST_STRATEGY is set."
+        "Collectfast failed to guess strategy, please make sure "
+        "COLLECTFAST_STRATEGY is set."
     )
diff --git a/collectfast/tests/no_preload_metadata.py b/collectfast/tests/no_preload_metadata.py
deleted file mode 100644
index 0228c4b..0000000
--- a/collectfast/tests/no_preload_metadata.py
+++ /dev/null
@@ -1,10 +0,0 @@
-from storages.backends.s3boto3 import S3Boto3Storage
-
-
-class NPM(S3Boto3Storage):
-    """
-    Dummy class for testing that collectfast warns about overriding the
-    `preload_metadata` attribute.
-    """
-
-    preload_metadata = False
diff --git a/collectfast/tests/strategies/test_guess_strategy.py b/collectfast/tests/strategies/test_guess_strategy.py
index c640f78..a1b4433 100644
--- a/collectfast/tests/strategies/test_guess_strategy.py
+++ b/collectfast/tests/strategies/test_guess_strategy.py
@@ -1,5 +1,7 @@
 from unittest import TestCase
 
+from django.core.exceptions import ImproperlyConfigured
+
 from collectfast.strategies.base import _BOTO3_STORAGE
 from collectfast.strategies.base import _BOTO3_STRATEGY
 from collectfast.strategies.base import _BOTO_STORAGE
@@ -24,7 +26,8 @@ def test_guesses_boto3_from_exact(case):
 def test_guesses_boto_from_subclass(case):
     # type: (TestCase) -> None
     case.assertEqual(
-        guess_strategy("collectfast.tests.boto_subclass.CustomStorage"), _BOTO_STRATEGY
+        guess_strategy("collectfast.tests.test_storages.boto_subclass.CustomStorage"),
+        _BOTO_STRATEGY,
     )
 
 
@@ -32,6 +35,20 @@ def test_guesses_boto_from_subclass(case):
 def test_guesses_boto3_from_subclass(case):
     # type: (TestCase) -> None
     case.assertEqual(
-        guess_strategy("collectfast.tests.boto3_subclass.CustomStorage"),
+        guess_strategy("collectfast.tests.test_storages.boto3_subclass.CustomStorage"),
         _BOTO3_STRATEGY,
     )
+
+
+@test
+def test_raises_improperly_configured_for_unguessable_class(case):
+    # type: (TestCase) -> None
+    with case.assertRaises(ImproperlyConfigured):
+        guess_strategy("collectfast.tests.test_storages.unguessable.UnguessableStorage")
+
+
+@test
+def test_raises_improperly_configured_for_invalid_type(case):
+    # type: (TestCase) -> None
+    with case.assertRaises(ImproperlyConfigured):
+        guess_strategy("collectfast.tests.test_storages.unguessable.NotAType")
diff --git a/collectfast/tests/test_storages/__init__.py b/collectfast/tests/test_storages/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/collectfast/tests/boto3_subclass.py b/collectfast/tests/test_storages/boto3_subclass.py
similarity index 100%
rename from collectfast/tests/boto3_subclass.py
rename to collectfast/tests/test_storages/boto3_subclass.py
diff --git a/collectfast/tests/boto_subclass.py b/collectfast/tests/test_storages/boto_subclass.py
similarity index 100%
rename from collectfast/tests/boto_subclass.py
rename to collectfast/tests/test_storages/boto_subclass.py
diff --git a/collectfast/tests/test_storages/unguessable.py b/collectfast/tests/test_storages/unguessable.py
new file mode 100644
index 0000000..b20a7c3
--- /dev/null
+++ b/collectfast/tests/test_storages/unguessable.py
@@ -0,0 +1,5 @@
+class UnguessableStorage:
+    pass
+
+
+NotAType = 1
diff --git a/setup.cfg b/setup.cfg
index da5b392..00b398f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -37,6 +37,10 @@ source = collectfast
 
 [coverage:report]
 omit = */tests/*
+exclude_lines =
+    pragma: no cover
+    # ignore non-implementations
+    \.\.\.
 
 [metadata]
 license_file = LICENSE