diff --git a/tests/custom.rs b/tests/custom.rs
index 4f9cb807..b085094b 100644
--- a/tests/custom.rs
+++ b/tests/custom.rs
@@ -16,6 +16,9 @@ fn len7_err() -> Error {
 }
 
 fn super_insecure_rng(buf: &mut [u8]) -> Result<(), Error> {
+    // `getrandom` guarantees it will not call any implementation if the output
+    // buffer is empty.
+    assert!(!buf.is_empty());
     // Length 7 buffers return a custom error
     if buf.len() == 7 {
         return Err(len7_err());