diff --git a/petsctools/options.py b/petsctools/options.py
index 413c2bd..1c6b078 100644
--- a/petsctools/options.py
+++ b/petsctools/options.py
@@ -133,8 +133,10 @@ def _warn_unused_options(all_options: Iterable, used_options: Iterable,
 def _validate_prefix(prefix):
     """Valid prefixes are strings ending with an underscore.
     """
+    if prefix is None:
+        raise ValueError("Cannot validate None as a prefix")
     prefix = str(prefix)
-    if not prefix.endswith("_"):
+    if prefix and not prefix.endswith("_"):
         prefix += "_"
     return prefix
 
diff --git a/tests/test_options.py b/tests/test_options.py
index 67527fc..a4d5130 100644
--- a/tests/test_options.py
+++ b/tests/test_options.py
@@ -84,18 +84,22 @@ def test_options_prefix():
     options = petsctools.OptionsManager({})
     assert options.options_prefix.startswith("petsctools_")
 
+    # User defined empty prefix
+    options = petsctools.OptionsManager({}, options_prefix='')
+    assert options.options_prefix == ""
+
     # User defined default prefix
     options = petsctools.OptionsManager({}, default_prefix="firedrake")
     assert options.options_prefix.startswith("firedrake_")
 
     # Explicit prefix overrides default prefix
     options = petsctools.OptionsManager({}, options_prefix="myobj")
-    assert options.options_prefix.startswith("myobj_")
+    assert options.options_prefix == "myobj_"
 
     # Explicit prefix overrides default prefix
     options = petsctools.OptionsManager({}, options_prefix="myobj",
                                         default_prefix="firedrake")
-    assert options.options_prefix.startswith("myobj_")
+    assert options.options_prefix == "myobj_"
 
 
 @pytest.mark.skipnopetsc4py