diff --git a/packages/testing/src/execution_testing/client_clis/ethereum_cli.py b/packages/testing/src/execution_testing/client_clis/ethereum_cli.py
index c2ca9d11d5..c15671519d 100644
--- a/packages/testing/src/execution_testing/client_clis/ethereum_cli.py
+++ b/packages/testing/src/execution_testing/client_clis/ethereum_cli.py
@@ -64,7 +64,7 @@ def __init__(self, *, binary: Optional[Path] = None):
             resolved_path = Path(os.path.expanduser(binary)).resolve()
             if resolved_path.exists():
                 binary = resolved_path
-        binary = shutil.which(binary)  # type: ignore
+        binary = shutil.which(str(binary))  # type: ignore
         if not binary:
             raise CLINotFoundInPathError(binary=binary)
         self.binary = Path(binary)
@@ -239,7 +239,7 @@ def is_installed(cls, binary_path: Optional[Path] = None) -> bool:
             resolved_path = Path(os.path.expanduser(binary_path)).resolve()
             if resolved_path.exists():
                 binary_path = resolved_path
-        binary = shutil.which(binary_path)
+        binary = shutil.which(str(binary_path))
         return binary is not None
 
     @classmethod
diff --git a/packages/testing/src/execution_testing/client_clis/tests/test_ethereum_cli.py b/packages/testing/src/execution_testing/client_clis/tests/test_ethereum_cli.py
new file mode 100644
index 0000000000..1770f74be3
--- /dev/null
+++ b/packages/testing/src/execution_testing/client_clis/tests/test_ethereum_cli.py
@@ -0,0 +1,29 @@
+"""Tests for the `EthereumCLI` base class."""
+
+from pathlib import Path
+
+import pytest
+
+from execution_testing.client_clis.ethereum_cli import (
+    CLINotFoundInPathError,
+    EthereumCLI,
+)
+
+
+class _MissingCLI(EthereumCLI):
+    """A CLI whose binary is guaranteed not to be on `PATH`."""
+
+    default_binary = Path("eels-nonexistent-binary-xyz")
+
+
+def test_is_installed_accepts_path_binary() -> None:
+    """`is_installed` handles a `Path` binary without raising."""
+    # `shutil.which` must receive a `str`; a `Path` crashes on some
+    # platforms (e.g. Windows under Python 3.11).
+    assert _MissingCLI.is_installed() is False
+
+
+def test_init_missing_path_binary_raises_cli_not_found() -> None:
+    """Constructing a CLI with a missing `Path` binary raises cleanly."""
+    with pytest.raises(CLINotFoundInPathError):
+        _MissingCLI()