diff --git a/client.go b/client.go
index f8a6b9c..e8b0420 100644
--- a/client.go
+++ b/client.go
@@ -161,7 +161,7 @@ func (c *client) Call(
 	}
 	procedure := spec.ProcedureForPath(procedurePath)
 	if procedure == nil {
-		return fmt.Errorf("no procedure for path %q", procedurePath)
+		return NewErrorf(CodeUnimplemented, "procedure unimplemented: %q", procedurePath)
 	}
 	data, err := marshalRequest(c.format, request)
 	if err != nil {
diff --git a/pluginrpc_test.go b/pluginrpc_test.go
index 0e409f6..75611e8 100644
--- a/pluginrpc_test.go
+++ b/pluginrpc_test.go
@@ -108,6 +108,25 @@ func TestEchoError(t *testing.T) {
 	)
 }
 
+func TestUnimplemented(t *testing.T) {
+	t.Parallel()
+	forEachDimension(
+		t,
+		func(t *testing.T, client pluginrpc.Client) {
+			err := client.Call(
+				context.Background(),
+				"/foo/bar",
+				nil,
+				nil,
+			)
+			pluginrpcError := &pluginrpc.Error{}
+			require.Error(t, err)
+			require.ErrorAs(t, err, &pluginrpcError)
+			require.Equal(t, pluginrpc.CodeUnimplemented, pluginrpcError.Code())
+		},
+	)
+}
+
 func forEachDimension(t *testing.T, f func(*testing.T, pluginrpc.Client)) {
 	for _, format := range allTestFormats {
 		for j, newClient := range []func(...pluginrpc.ClientOption) (pluginrpc.Client, error){newExecRunnerClient, newServerRunnerClient} {