diff --git a/src/hyperlink/hypothesis.py b/src/hyperlink/hypothesis.py
index 4ab987e..3be47da 100644
--- a/src/hyperlink/hypothesis.py
+++ b/src/hyperlink/hypothesis.py
@@ -31,7 +31,7 @@
 
     from . import DecodedURL, EncodedURL
 
-    from hypothesis import assume
+    from hypothesis import reject
     from hypothesis.strategies import (
         composite,
         integers,
@@ -137,7 +137,7 @@ def idna_text(draw, min_size=1, max_size=None):
         try:
             idna_encode(result)
         except IDNAError:
-            assume(False)
+            reject()
 
         return result
 
@@ -198,7 +198,7 @@ def hostname_labels(draw, allow_idn=True):
         try:
             check_label(label)
         except UnicodeError:  # pragma: no cover (not always drawn)
-            assume(False)
+            reject()
 
         return label
 
@@ -318,4 +318,8 @@ def decoded_urls(draw):
         Call the L{EncodedURL.to_uri} method on each URL to get an HTTP
         protocol-friendly URI.
         """
-        return DecodedURL(draw(encoded_urls()))
+        encoded_url = draw(encoded_urls())
+        try:
+            return DecodedURL(encoded_url)
+        except UnicodeDecodeError:
+            reject()
diff --git a/tox.ini b/tox.ini
index 8865d17..56caefe 100644
--- a/tox.ini
+++ b/tox.ini
@@ -93,7 +93,15 @@ basepython = {[default]basepython}
 skip_install = True
 
 deps =
+    # Pin black and all of its dependencies so an incompatible release of one
+    # of them doesn't break this environment.
+    appdirs==1.4.4
     black==21.7b0
+    click==7.1.2
+    mypy-extensions==0.4.3
+    pathspec==0.9.0
+    regex==2022.8.17
+    tomli==1.2.3
 
 setenv =
     BLACK_LINT_ARGS=--check