diff --git a/library/core/src/iter/adapters/zip.rs b/library/core/src/iter/adapters/zip.rs
index 98b8dca961407..9f98353452006 100644
--- a/library/core/src/iter/adapters/zip.rs
+++ b/library/core/src/iter/adapters/zip.rs
@@ -198,12 +198,13 @@ where
                 Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
             }
         } else if A::may_have_side_effect() && self.index < self.a.size() {
+            let i = self.index;
+            self.index += 1;
             // match the base implementation's potential side effects
-            // SAFETY: we just checked that `self.index` < `self.a.len()`
+            // SAFETY: we just checked that `i` < `self.a.len()`
             unsafe {
-                self.a.__iterator_get_unchecked(self.index);
+                self.a.__iterator_get_unchecked(i);
             }
-            self.index += 1;
             None
         } else {
             None