diff --git a/src/parser/parsers.h b/src/parser/parsers.h
index 48f65c497e0..c90e2c01d1f 100644
--- a/src/parser/parsers.h
+++ b/src/parser/parsers.h
@@ -1005,6 +1005,13 @@ template<typename Ctx> MaybeResult<> foldedinstr(Ctx& ctx) {
       auto inst = plaininstr(ctx, std::move(info.annotations));
       assert(inst && "unexpectedly failed to parse instruction");
       CHECK_ERR(inst);
+      // We have already parsed the instruction, so we generally know where it
+      // ends. But there may have been some invalid extra immediates (e.g.
+      // invalid memory indices) that we only realize are invalid now that we've
+      // parsed the instruction for real.
+      if (ctx.in.getPos() != *info.end) {
+        return ctx.in.err("expected end of instruction");
+      }
       assert(ctx.in.getPos() == *info.end && "expected end of instruction");
       continue;
     }
diff --git a/test/lit/parse-bad-optional-memidx.wast b/test/lit/parse-bad-optional-memidx.wast
new file mode 100644
index 00000000000..57438d6d2fd
--- /dev/null
+++ b/test/lit/parse-bad-optional-memidx.wast
@@ -0,0 +1,17 @@
+;; Regression test for a parser bug where the invalid memory index followed by
+;; another immediate caused an assertion failure.
+
+;; RUN: not wasm-opt -all %s 2>&1 | filecheck %s
+
+;; CHECK: Fatal: 12:22: error: expected end of instruction
+
+(module
+ (memory 1 1)
+
+ (func $v128.load16_lane1 (param $0 i32) (param $1 v128) (result v128)
+  (v128.load16_lane 1 0 ;; invalid memory index
+   (local.get $0)
+   (local.get $1)
+  )
+ )
+)
\ No newline at end of file