diff --git a/src/combinator.rs b/src/combinator.rs
index 12c25692..70617eec 100644
--- a/src/combinator.rs
+++ b/src/combinator.rs
@@ -2008,6 +2008,8 @@ where
     #[inline(always)]
     fn go<M: Mode>(&self, inp: &mut InputRef<'src, '_, I, E>) -> PResult<M, ()> {
         let mut state = self.make_iter::<Check>(inp)?;
+        #[cfg(debug_assertions)]
+        let mut first_loop = true;
         loop {
             #[cfg(debug_assertions)]
             let before = inp.cursor();
@@ -2020,11 +2022,15 @@ where
                 Err(()) => break Err(()),
             }
             #[cfg(debug_assertions)]
-            debug_assert!(
-                before != inp.cursor(),
-                "found SeparatedBy combinator making no progress at {}",
-                self.location,
-            );
+            if !first_loop {
+                debug_assert!(
+                    before != inp.cursor(),
+                    "found SeparatedBy combinator making no progress at {}",
+                    self.location,
+                );
+            } else {
+                first_loop = false;
+            }
         }
     }
 
diff --git a/src/lib.rs b/src/lib.rs
index d361d997..c155f11e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -3861,6 +3861,17 @@ mod tests {
         )
     }
 
+    #[test]
+    fn separated_by() {
+        use crate::{error::Simple, extra};
+
+        let parser = just::<_, &str, extra::Err<Simple<_>>>("a")
+            .or_not()
+            .separated_by(just("b"));
+
+        assert_eq!(parser.parse("bba").into_result(), Ok(()));
+    }
+
     #[test]
     fn zero_size_custom_failure() {
         fn my_custom<'src>() -> impl Parser<'src, &'src str, ()> {